The limits are provided through the Digital Ocean Documentation: Firewall Overview, although I have included the relevant ones below for your information:

Limits

• You can have a maximum of 10 Droplets per Firewall and 5 Tags per Firewall. If you have more than 10 Droplets which need the same firewall, tag the Droplets with the same tag, then add that tag to the firewall.
• Each Firewall can have up to 50 total incoming and outgoing rules.
• The Cloud Firewall affects both public and private network traffic, meaning rules specific to either will need to specify the public or private IP range.
• The Cloud Firewall supports ICMP, TCP, and UDP protocols only.