Master/slave with multiple OpenVAS scanner

I have a master/slave setup and running with a local and a scanner (running on the master) and an additional OpenVas scanner. So now I have two OpenVAS scanners in the scanner configuration. When I try to add a third OpenVAS scanner, I cannot select type OpenVAS. Now it only gives me the option of GVM and OSP scanner types, but not OpenVAS.

Is there some kind of limitation on the number of OpenVAS slaves I can add to GSE?

running gvm 20.08