How many stamps do you need for 12.5oz?

The first ounce is 45 Cents.

Each additional ounce or fraction (12 in this case) would cost 20 cents each for a total of $2.85. If it is a 'flat' there is a surcharge for over-sized envelopes and 'non-machinable' envelopes.

If it is a parcel rather than a large letter, it would only be 17 cents each.