Hi Chloris,
You cannot know how many submissions you have per group unless you download the results file. The preview of the results (accessible by clicking the “…” button to the right of the “Results” button) will tell you how many submissions you have received so far in total at the corresponding link — just make sure you refresh the page to get the latest information
You cannot set a limit to the number of submissions on the farm, as multiple participants might start the experiment in parallel and a subset of them might of might not complete it: there is no principled way of knowing in advance how many submissions you will eventually receive. One common way of handling this point, assuming you are using a crowd-sourcing platform, is to open a limited number of slots on that platform, and open new ones as needed
Jeremy