ALet say there is a cupcake recipe for 24 pcs. However you want to make 50 pcs, how would you change the recipe?

Double the recipe and just make them slightly smaller so that you get 50, instead of 48, out of the mix,
 

AIt means you can double any recipes?

If you want to be really precise, and this is probably only feasible if you're working in grams, divide each of your recipe quatities by 24 (to get the recipe amount for a single cupcake)  then multiply that amount by 50.  Use a pocket calculator or a spreadsheet. 
 

fine in theory but will get messy with the egg amounts
 

You're right cazza1, but it's not as bad as you might think.  Egg quantities have a certain amount of leeway.  I buy eggs from a local farm and they come unsorted in varying sizes but the average weight (unshelled) is 54g.  I worked that out when making a huge wedding cake using about 30 eggs... So if I work out egg quantities by weight I can usually get it very close by selecting a larger or smaller egg for the last one...  Also with most cake recipes if you're a bit under, it's OK to make up the egg weight with milk or water.   Yes I'm, a bit OCD about it...
 

for any fraction of an egg

beat it a bit and just use half or a third or whachu need