When stuck on estimating the lowest number of resources a project can take, one way to get unstuck is to mentally apply zero resources of a particular type to accomplish a desired deliverable. When that number seems patently absurd, I then go to the next small incremental quantity of that applied resource. Does that number still seem absurd? Then I go up again.

Very quickly, with simple mental gymnastics, I can find the absolute "bottom floor" of the smallest resource quantity I can apply after which a project does not deliver on the basics.