According to the rules:
"You must have at least 13 and no more than 17 hitters.
- At least two of those hitters must be eligible to play catcher."
There's a team in my league that seems to have gotten away with just one:
http://fantasygames.sportingnews.com/stratomatic/team/team_other.html?user_id=205400&stats=actual
Is that rule not actually enforced?