How many friend invites can I send out? What is the limit? And what do I need to do to stop getting the error "Couldn't send friend invite, but try again"
I was checking for you in the Stadia helpcenter, but was unable to find any friend-invite-limit, so either there is no limit or there is a limit which is not shown anywhere there.
Regarding the error I would recommend to try a restart again and then see if that solves the issue, otherwise it might be an idea to contact the Stadia support as they might know exactly what you need to do with that specific error message.