I want to buy a Mikrotik RB750GR3. Can anyone tell me how much memory can RB750GR3 support? Will it be ok for 50-100 users? TIA

It depends. What Internet bandwidth do you have, up- and downstream? What will your firewall look like? To summarize, what are your requirements besides number of users!

My bandwidth will be 100mbps downstream and 10mbps upstream. I will run some simple queue with basic firewall rules. Basically it will be a mini-ISP setup.

WTF

how many topics do you plan to open to ask the same thing ??

RB750GR3 handles 100/100 fine. For queues I can't tell.