How much buttercream do I need for 60 cupcakes? Cupcake size traditional 3oz. well

I rarely do cupcakes, so have no idea.  It would also depend upon how you planned to apply the icing.  Why not try one, weigh it and multiply by 60.

A rule of thumb is 1batch of icing for every cake mix used —-each mix usually makes 24 cupcakes soooo I suggest 3 batches of icing that uses 2# of sugar. It will depend on what tip is used & how hard you squeeze!   Will they be all alike I color & style?  I’d make 3 but have ingredients On hand for at least one more batch.  If you end up w/ extra leftover it keeps in the frig & freezes great for any future baking:)