If your nights are that cold you will need quite a few bees in there to keep the brood covered. Because you are making up the nucs from hives and the brood, at first at least, may not be the best shape for the design of the nuc, you'll need the bees that are on the combs when you make them, plus maybe a shake from another comb. Later in the season when things are warmer you need not be quite so fussy.

What I do is gauge it by the brood comb, a comb with only a small circular patch of brood will not need as many bees as one with a full comb of brood.