What is a good number? 22: 2 lines A, 2 lines D, 3 lines M, 1 G
Whether at youth ages or during showcase years, players who can handle more than one position will always bring an extra dimension to a team. Younger age groups will see a player with more playing time if he/she can handle a midfield/attack or attack/defense role rather that just playing "right side attack" or "left side midfield". Yes, long pole versus short pole debates complicate this picture, but several players will own both which might help the situation.
College coaches will often seek players who can handle more than one position and can be moulded into the coach's system of play. In addition to ability and skill, field flexibility is a major "plus" for any college-bound player.
BOTC mentions this as it becomes easier to manage with an 18 player roster (hence more playing time per player) when you have that type of flexibility on the field.
Ideally, a goalie who can also double in the field is your best option for a second keeper. Carrying only one net minder comes with obvious risks although that keeper will garner all playing time and not have to worry about sharing the net. Making your keeper part of the defensive unit as opposed to just being the keeper is very important.