If you're worried about more than a couple of vlans on a firewall, I strongly recommend putting in an actual layer 3 switch to handle your routing needs. I understand idle curiosity, but this question sounds like the lead-in to a design choice you'll regret.
I am going to copy @Nash comment "Configurable count is really different from supportable." as it makes perfect sense! While there are no hard limits, I would like you to be aware of two limits though:
1. UI supports only 256 L3 interfaces on MS switch series.
2. You can only have a maximum of 255 VLANs configured on MXs in HA. - Dashboard will not stop you from going beyond 255 but will break things if you do. This is a known bug and dev team is looking into a long term fix on this one.