The Security Appliances do not limitation regarding the number of VLANs allowed. That will depend on the memory and the CPU utilization and other features you are running as well (content filtering, threat protections, VPN tunnels, etc).
What kind of issues are you having in those VLANs:
- Is the MX not providing IP addresses?
+ Did you take pcaps to see if there are DHCP Discovers and Offers?
- Does the MX say that there are not available IP addresses?
+ Did you check the DHCP section in the Appliance Status and the IP reservations in the DHCP section?
- The MX does not provide IP leases at all?
+ Did you check the DHCP configuration for that VLAN?
You can find more information about the MX84 and the comparison against other MXs here: