I have no idea what type of pancake you're making, but I had answered your question for chemically leavened pancakes in case someone stumbled upon that question (even though the earlier question was specifically for crêpe like pancakes.)
If you're using baking soda as a leavening, it'll have already given its all by the time you cook it hours later ... but you'd have to add baking powder to compensate when it's time, as there might not be enough acid left for baking soda. Baking powder in the batter isn't as much of a problem if it's double-acting, where it'll give some leavening when it gets wet, and again as it gets warm. Of course, you don't want to add too much baking powder, as it can give a metallic taste.
It's also worth mentioning that it is possible to halve an egg; in this case, if you're going to be using the other half of the mix in a day or two, it might be easier to measure by mixing all of the wet ingredients together and then use half.
