We have about 4 substitutes at my school. One of them is covering a maternity leave since the long term sub was fired. Teachers at my school have a prep period and a common planning every day. If there are a lot of teachers out, they have a teacher cover during their prep or common planning. If you cover during prep, you take your prep during your common planning, so you never skip your prep. The contract changed a bit this year and they aren't supposed to pull teachers to cover as much. I've had to cover about 10 times this year.