Slices with single numbers on them are the standard pizza types. The number is both the quantity of that type of slice in the game, as well as how many points a player will receive at the end of the game if they have collected the majority of those slices.
Most numbered slices have pepperoni on them. Eating these slices results in + 1 point per pepperoni eaten.
Some slices have anchovies on them; if these slices are collected the anchovies are subtracted from your score. If slices with anchovies are eaten, the anchovies are not counted against you.