bandwidth can be limited in terms of how much speed(mbps/gbps) can be used by each user and not by how much mb/gb can be downloaded by each user.
To limit the speed, you can use traffic shaper objects(policy and objects>object>traffic shaper>create new ) and specify the type(per IP or shared) and then specify the maximum bandwidth and apply the same on the respective policy(VLAN>WAN).
here is few more information about the 2 types of shapers
1: Shared :-
shared gives you 2 options while adding the object
* Per Policy
* For All Policies Using This Shaper
Needs to be applied on one policy and all the users connecting through that policy will share the mentioned amount of bandwidth.
For All Policies Using This Shaper:-
same object can be applied on multiple policies. and all the users connecting through all those policies together will take maximum that much of bandwidth .
2: Per IP:-
If you apply this on a policy , then each user connecting through that policy can take the mentioned bandwidth.
SO if you have mentioned 200 kbps , and applied in on LAN to WAN policy, lets say 20 users are currently connected through that policy, then each of them will take 200 Kbps max.