Greetings!
At the company where I work we undertook a project of test-covering customers with p2mp. For this goal we used RBSXTG-5HPacD-SA as an access point and RBSXTG-5HPacD client-side.
The maximum distance from an access point to a user is about 600 meters.
We're currently having problems with the number of users connected to one AP. Once we reached about 15 connected users we began having troubles guaranteeing quality services thru the wireless network.
Thus, my question is: What is the maximum number of users for each AP to ensure 20Mbps download and 10Mbps upload for each client?

Unfortunately with wireless, there really isn't a simple answer to this question because it depends on lots of things.

Of course a clear RF environment is a big key, but there are other factors, such as what type of traffic are you handling. If you're handling VoIP or some other application that has a high packets/second count, this can eat through your total capacity much faster than you might think - bandwidth isn't the only concern. With wireless, time is really key, because only one device can be transmitting at any given moment.

Another troublesome issue is if you have lots of good links, but one client has a poor signal - the AP has to transmit this station's packets slowly, which chews up precious air time. If a user is using a basic rate like 6Mbps modulation, then the time it takes for 4KB of data to reach this customer could have sent 200KB to other customers at 300Mbps modulation rate.

So one 6Mbps link is like having 50 300Mbps links from a density planning perspective.

Also - broadcasts / multicasts on the sector take longer because the sector uses a base rate to send these packets. It has to transmit no faster than the slowest modulation rate on the sector, or else the transmission might get corrupted on the slow link.

In practice, we had some sectors that could only handle 3 or 4 links, while others worked just fine with 15 or more clients. (these were Ubnt airmax sectors, operating in a congested RF environment - dual chain 20Mhz channel - 5Ghz band)

Thank you for your answer.
Is there any sort of a solution that can be recommended for our situation?
Would it be a good idea to separate troublesome links from the good ones to a second AP? If so, what sort of a configuration is recommended each AP? Our current traffic type is normal internet and iptv via unicast, but we also intend on adding voip.

Anyone?

Sorry "Bro", but there is no magic trick for your situation. Enjoy with 15 client's and 20Mb that you have because it's still quite a lot.
You should use nv2 and AMPDU for traffic prioritization, QoS for services and wait for AC fixes in RouterOS