Basically depending on what you set your limit on. Right now it is set to 50 KiB which is the size of each PCQ-queue. you mutiply that by the number of individual queues you require (concurrent users you expect to have connected) ie 80 which gives you the pcq-total-limit= 4000KiB . you then need to make sure you have enough ram. Lets say PCQ-TOTAL-LIMIT=x
RAM required = x*(2000Byte+200Byte) (2000Byte buffer for 1 packet. 200 Byte service data for 1 packet)
Ram required = 4000KiB*2.2 = 8800 Kib = <8.8 MB Ram required if you have 80 people concurrently online at one time. If you only have a few customers then this is not going to affect much but some of the larger ISP’s will be taking this into account. It is covered under this page. http://forum.mikrotik.com/t/pcq-limit-total-limit-settings/12002/1 If you look at https://mum.mikrotik.com//presentations/US08/janism.pdf There is more detail.