Hugleo,
There is "NO" hard coded limit for firewall filters in MX Series and the usage is based on the number filter in use (binded to interfaces).
The memory for Filters is allocated from MPC/FPC PFE. The following command indicated the firewall filters per FPC.
The summary keyword is hidden:
#run show system resource-monitor summary
Slot # 1
Client allowed : Yes
Service allowed : Yes
Client denied : 0
Service denied : 0
Heap memory used : 207534120 In % : 6
Filter memory IFL memory Expansion memory
PFE # used | % used | % used | %
0 23190992 29 267792 0 0 0 <<<<<<<<<<<<<<<<