There are 6 members on my trello board. There is limit 10 participants. But I can't add new ones and get an error that I have more than 10 participants.
I am not a Trello user myself, but I've seen this question pop up quite often.
The answer is usually in the fact that your board might not have 10 users yet, but the limits are being checked on workspace level (so it looks at all the boards that exist within trello.com/w/<name of your workspace>). Which means that you'll have to open your workspace Members settings and see if there are no more than 10 users there.