It's fine - really no different than cake. In fact my crowd prefers cupcakes. There are calculators out there. When I bake for events I use 1.5 as my standard usually and rarely have more than 6 to 12 left over.

If cupcakes are your ONLY dessert offering, you may want to consider 2.0 (maybe 1.75). If you plan to have other desserts available, you could consider 1.25 or 1.0. There are a few factors that come into play (full meal vs. Cocktail or dessert reception only, other desserts being served, etc).

If cupcakes are in lieu of cake, definitely have enough for everyone to have one, even if there will be other desserts. I've seen some try to get by cheap ordering enough for 75 percent figuring not everyone will have one or prefer one of the other dessert options, but when it is taking the place of the actual wedding cake you should definitely plan on everyone. I probably wouldn't go less than 1.5 though.