Tested in VyOS 1.4-rolling-202112160318 and VyOS 1.3.0
There is currently a limit of 9999 rules.
Given the common practice of creating rules in increments of 10 (or even 100), that gives us 1000 (or only 100) rules. We have over 1300 rules in our existing Firewall. When trying to migrate to VyOS, we ran into this limitation.
If this restriction can be changed, it would be very useful for some installations.
For example, 999999 would be great