I would be tempted to write a little script to create, say 256 group policies, and then delete them. That will give you a quick taste of weather it can be accepted.
I would then try writing a script to create say 256 VLANs (and then delete them). This will give you a taste of weather the chipset can actually support that many.
I think at 150 you would be pretty safe.
I'm guessing the scaling issues would come from:
* Being able to manage that many group policies in the dashboard
* The additional CPU load on the MX to compile and process that many rules. I would think applying them only based on VLAN would make things much easier.
* The chipset inside of the respective MX being able to support a large number of VLANs.