I am running Microtik Router model RB1100AHx2(powerpc) builtin RAM 2gb, My question is: how many active hotspot clients can it handle with good performance?

Level 6 is unlimited

Thanks for the reply but as its the limit of the license, can the router actually support unlimited no of users logging in and most of the time active, let's say 1000 users at a given time.

Specify limits for a good performance is not possible once that you have many other things running in a router.
You can have firewall rules using part o CPU, and then your limit of Hotspot users can be lowest. Otherwise, how much bandwidth each Hotspot will have? The performance will be different in a router with 1000 Hotspot users with 512k/512k, and 1000 users with 10M/10M.

All depends that what you are running on a router in general. For an example, I have a customer running Hotspot server in a CCR1036, with about 1200 users + firewall rules, total throughput about 100 Mbps and the CPU runs at 6~10%.

let's say 500 active clients are connected and each is allowed a limit of 1Mbps on average, and there are no firewall rules, will the aforementioned router still be at good health
What's the red sign for the processor of a router ? utilization above 50 or 75?

i would like also to learn what is the "red sign" for cpu utilization for mikrotik routers? for example rb2011UiAS-2HnD