Hi,
Like ASA supports 1024 vlan interfaces there is no such doc which tells how many interfaces are supported in FMC because we have reported a couple of cases where in going above 150 interfaces the FMC does not display them properly.
Doc bug has been filed for it : https://tools.cisco.com/bugsearch/bug/CSCuw53314/?reffering_site=dumpcr
Also check this https://tools.cisco.com/bugsearch/bug/CSCut40770/?reffering_site=dumpcr
As a workaround you can add the zones under :
Objects -> Object Management -> Security zones -> Add security zone
Regards,
Aastha Bhardwaj
Rate if that helps!!!