Trying to display the number of opportunities depending on various conditions/filters with a rollup field, these opportunities aren't linked to my entity.

I have been trying to do this without using js, I think it won't work with a rollup field because opportunities aren't linked to my entity, and I didn't find the right function to do it with a calculated field(I think it's impossible with a calculated field)

I would like to either count opportunities applying directly my conditions/filters on all opportunities, or just get the number of rows in a custom opportunities view.

Can someone help me?

Thanks, Vincent

Edit : One way to do it could be linking all opportunities existing to my entity, is it possible to do so?