(Jeremy is traveling right now and may not be able to answer before a while, so I’m jumping in.)
There unfortunately is no fool-proof way of ensuring equal number of submissions across lists in an automated way. The only method giving you full control is to post separate experiments on Prolific (with restrictions set to allow people not to take more than one) with urls to your experiment that hard-code the group/list-assignment, in the way Jeremy described above:
“Circulate the link to your experiment ending with server.py?withsquare=0 to some participants, with server.py?withsquare=1 to some other participants, and so on, until you get 10 submissions for each list”
Then you can make sure that each list gets to the desired number of participants.
Hope that helps,