It is one large egg in a batch of pastry to make four 9.5" shells. In practice I use one medium egg (I get medium/small eggs from the farmer's market) and cut the recipe in half to make two, and I sometimes hold back a spoonful to use as egg wash on the bottom. The recipe I use is from Healy and Bugat's Mastering the Art of French Pastry (although I have also used Julia Child's recipe) and for four shells it uses 1 pound of butter, 650g/4.75 c flour, 4 tsp sugar, and 7 Tbsp cold water (I usually add more to help things cohere, but make sure to let the dough rest in the fridge to relax the gluten.)