Sometimes there is a glitch and you end up in a race with one racing member too many or too few. It may be because someone opted in/out quite shortly before the start of the regatta.
In some cases you may be suddenly placed in a different race group with the proper number of racing members. Try to contact customer support: Tap on the cog wheel in the top left, then on the orange speech bubble. Then tap on Contact in the top right. Choose the subject „Regatta“ and explain the problem. You will get an automatic answer immediately. Very important: Reply to that answer and refer again to your first message. After a few days somebody should answer you and hopefully be able to help you. Good luck.