can mikrotik router dynamic limit user bandwidth base on active user?
let say,
if 1pm have 50 active user using internet, 20Mbps / 50 = 0.4Kbps
4am have 10 user using internet, 20Mbps / 10 = 2Mbps

can mikrotik do like that?

use pcq.

http://wiki.mikrotik.com/wiki/Manual:Queues_-_PCQ

any advice/example how we wil do that?

google it dude simple queue with pcq

Hello

You can find examples here http://wiki.mikrotik.com/index.php?titl ... edirect=no

but target-addresses should be just target also parent=global-in must be parent=global