lakshya
The limits are five times the average of all users of the service. Specifically, this means that no more than 250 users per forum are supported.
...and more than ~ 50 visitors active at the same time, interacting with the forum by viewing or participating in discussions.
I don't know user count of your forum. If it's not that big, the error has nothing to do with user count as it should be per user, and it does not depend on visitor count. I wrote the part about user limits because I took in account the possibility that 429 can be triggered when there are too many requests from all users, but on second thought I remembered that PHP-FPM handles it per client, so what I wrote was pretty much a nonsense
Put simply, don't be so fast between clicking, posting discussions, page reloads and similar stuff and you're fine.