Sealed Envelopes

A set of ten cards, each showing one of the digits from 0 to 9, is divided up between five envelopes so that there are two cards in each envelope. The sum of the two numbers inside it is written on each envelope:

five envelopes with the numbers 7, 8, 13, 14 and 3 written on them (one number per envelope)

What numbers could be inside the “8” envelope?

Key Questions: What are the possible ways of making the numbers on the envelopes?
Which number has the fewest possible combinations?