I’m assuming that you were told this by a friend or potential guest that tried the search, not by Airbnb.
Before doing anything, verify that there really is a problem by going to the desktop version of Airbnb, putting your browser in Private mode, and trying the different guest number searches yourself using your guest account. If you can get that to work, try the same search on your iPhone (which uses a different version of Airbnb).
If you verify that there is indeed a problem, you can make sure you have your max guests set correctly, but you’ve probably already verified that.
Next step is to call Airbnb. I’ve had the best luck calling CS during US business hours, or any country’s business hours in which CS may have engineers accessible that actually work on the Airbnb software.