This is pretty straightforward - the more survey panels you join, the more survey invitations you’ll get. Each survey panel hosts a limited number of survey projects for each specific country, which in turn, affects the number of survey invitations you’re able to get. That issue can be resolved if you register with other legitimate survey panels as well, increasing your source of paid survey opportunities.
The main takeaway? Join more survey panels, but make sure they’re legitimate so you don’t end up wasting your time and effort!