What would you use as AP to serve 200+ users ?
Switch chip/CPU is the same and specs are very similar....
Radio coverage differences ?

Now I'm no expert here but 200+ on a single AP sound like trouble if you ask me.

Where is this to be used? large room or outside? Regardless of this having so many users on one (or in this case two radios) is a lot I would claim.

Absolutely right.
200+ users for 1 AP is madness (or recipe for self-torture ? Some may like it ...).

Powering
Number of DC inputs 2 (DC jack, PoE-IN)
DC jack input Voltage 18-57 V
Max power consumption 36 W
Max power consumption without attachments 11 W
Cooling type Passive
PoE in 802.3af/at

PoE-out
PoE-out ports Ether2
PoE out Passive PoE up to 57V
...

https://mikrotik.com/product/cap_ax

Besides the obvious (aesthetics, I/O and price), on paper it looks like the hAP-ax3 give better wireless performance. Interestingly, the hAP ax2 also appears to perform better than the cAP ax which I find strange considering the much bigger size of the cAP ax. Mabye someone can confirm?

The question you're actually asking here (without necessarily knowing it) is about calculating available WiFi airtime. There are only so many microseconds in a second, and each device on the network (which includes the AP) cannot transmit at the same time. So, each time anything is transmitting, it's consuming some of the available time. The longer they take to transmit whatever they're doing, the less time is available for other clients.

The things which factor into how bad this is will depend on a chunk of factors, including:

• Number of SSIDs on the AP
• Channel width of the radio
• Channel width used by each client
• Number of antennas on the AP and on each client
• Whether each transmission uses 802.11a, ac, ax, or n
• The number of clients and much of the above
• The MCS rate used to transmit by the AP to each client
• The MCS rate used by each client to transmit to the AP
• The number of retransmits required due to interference, poor signals, etc.

How well or badly connections to the AP suffer congestion collapse depends on the answers to the above. And these answers are constantly changing all the time as clients come and go off the network, move closer or farther away from the AP, and if other things start transmitting on the same frequency that aren't related to this AP, perhaps not even related to WiFi (common on the 2.4Ghz band).

Going back to your original question, you're only asking the basics about antenna performance and coverage. These are valid questions, but they're not enough. Lastly, I will add that 200 clients on a single AP will likely lead to congestion collapse. 802.11ax can help due to the use of MU-OFDMA and RRUs but it's not a silver bullet, particularly as many clients won't even be using 802.11ax.

Naturally, an AP with two radios (2.4 and 5Ghz) can spread out their load across the two frequencies, but it too is no silver bullet and its effectiveness depends on whether you have enabled RRM and WNM, and whether each client even cares about that data.

Thanks for replies,
Well, actually an hotspot is managing about 300 wifi users on 50mt coverage radius, over two old asus rt ac1200 wifi router (how bad ? they seems don't care about...)
They asked me for replacing Asus with AX devices: since customer is more worried by cost rather than performance , it is enough a hAP-ax3 performs equallly or sligtly better than asus devices, that's my concern.
Obviously the whole system should be revised for proper performances....

Thanks for replies,
Well, actually an hotspot is managing about 300 wifi users on 50mt coverage radius, over two old asus rt ac1200 wifi router (how bad ? they seems don't care about...)
They asked me for replacing Asus with AX devices: since customer is more worried by cost rather than performance , it is enough a hAP-ax3 performs equallly or sligtly better than asus devices, that's my concern.
Obviously the whole system should be revised for proper performances....

Is it possible to do roaming between asus and mikrotik? If yes then maybe you could use both on different channels.

Is it possible to do roaming between asus and mikrotik? If yes then maybe you could use both on different channels.

As long as all security settings (and SSID) are equal, you should be able.

Is it possible to do roaming between asus and mikrotik? If yes then maybe you could use both on different channels.

As long as all security settings (and SSID) are equal, you should be able.

Just beware of what "roaming" means. In answer by @erlinden, "roaming" means that station (wifi client) will be able to reconnect to AP (made by different vendor) when it decides to do so and with minimum handshake. This usually means short interruption in traffic flow (duration length in order of seconds) which allows ongoing connections to survive with a short pause.

Then there's something called "handover" in the world of mobile networks. In this case network infrastructure actively collaborates (or even drives) in station mobility, traffic flow never stops and the whole process is unnoticable for users. In WiFi workd this kind of mobility is collectively enabled by standards 802.11 r/k/v. Since this procefure requires coordination between old and new AP, it's usually (always?) constrained to APs from same vendor (in case of MT even same generation).
According this meaning of "roaming", roaming between MT and Asus is not possible.

Since this procefure requires coordination between old and new AP, it's usually (always?) constrained to APs from same vendor (in case of MT even same generation).
According this meaning of "roaming", roaming between MT and Asus is not possible.

This is true for hAP ax³ and Asus RT-AC1200. I think, however, it is not for MT and Asus in general.

To my knowledge, 802.11k/v/r support can be enabled on pretty much any piece of junk by the means of utilizing OpenWRT. It does not currently support hAP ax³. (If I were to guess, it's probably because they don't support ROS v7 bootloader either) But it does support hAP ac³ and Asus' RT-AC1200.

As for how well it will actually run... Uh... That I have no experience with. And after my last experience with OpenWRT, particularly with Apple devices... I am afraid of no man, but that thing, it scares me.