I would also make sure you are running a current stable or better firmware.
Apart from the issues, the others have mentioned to do with bandwidth and clients, I would not run 50 clients on an MR33/MR36 (unless they are doing nothing) because I doubt they have the CPU and DSP grunt to process that much control traffic (associations, disassociations, roaming events, etc.). If everyone was connected and no one moved and the WiFi network was perfectly stable they might cope - but that is just not reality. I could easily see them becoming unstable at 50 devices.
The MR44 might be borderline.
The MR53 should have no problems.
There is a reason why APs like the MR53 cost a lot more, apart from their radios - they have a much gruntier CPU and DSPs, and more RAM.
As a guide; if I was speccing it out - and it was quite a bit of area to cover, I would use twice as many MR36s as opposed to MR56s to compensate. In a smaller space, I would prefer to halve my cabling and switch ports and would go to the MR56s as the overall solution would end up cheaper. For MR36s, I aim to target no more than 20 clients per AP.
This is my personal opinion. Others may have different thoughts.