Scale any ingredient quantity up or down for a different number of servings.
Scaling ingredients is proportional: multiply each quantity by the target servings divided by the original servings. Halving, doubling and tripling work cleanly for flour, sugar, liquids and most solids. The arithmetic is the easy part, and it is also the part that misleads people into scaling everything.
Several things do not scale linearly, and ignoring that is why a doubled recipe can fail. Cooking time barely changes for the same-sized pieces but changes considerably for a deeper pan or a larger roast, since heat has further to travel — a doubled cake batter in one deep tin needs longer at a lower temperature, not double the time. Salt and strong spices often need slightly less than proportional amounts, because perceived intensity does not rise in a straight line. Leavening agents and yeast usually scale, but a much larger dough ferments differently. And pan size should scale by area, not by a linear dimension: doubling both sides of a tin quadruples the area, so a doubled batter needs a tin with roughly double the area, not double the width.
Rarely. Time depends mostly on thickness and pan depth rather than total quantity. Same-sized pieces cook in similar time, while a deeper pan needs longer at a lower temperature so the centre cooks before the outside burns.
Usually slightly less than double. Perceived intensity does not increase in a straight line, so scaling seasoning proportionally often overshoots. Scale to around three-quarters, taste, then adjust upward.
Match by area or volume, not by width. Doubling both dimensions of a tin quadruples its area, so a doubled recipe needs roughly double the area — a modestly larger tin, not one twice as wide.
Beat the egg, then measure by weight or volume and use the fraction you need. A large egg is roughly 50 grams without the shell, so half an egg is about 25 grams of beaten egg.