What is the maximum limit of linode instances to which we can attach a single firewall?

While there is no hard limit on the number of compute instances which can be associated with a single Cloud Firewall Rule, the following limitations would impact the number individual compute instances which would be able to be associated with a single firewall rule:

• A maximum of 255 IP addresses (and ranges) can be added to each Cloud Firewall rule.

• All IP addresses and IP Ranges must be formatted correctly, or changes will be unable to be saved.

• A maximum of 15 ports (and port ranges) can be defined on each Cloud Firewall rule.

These, and all other Cloud Firewall limitations can be found in the documentation below:
https://www.linode.com/docs/products/networking/cloud-firewall/#limits-and-considerations

-Micah