The indoor track season is only two months long. I think there are 36 spots for boys and 36 for girls, but some kids will not be selected for meets. As the previous poster noted, it is competitive to get on the team. I think indoor track has slightly fewer kids trying out, due to the short season.
The indoor track team practices twice per week after school. Practices are often outdoors, although they are sometimes inside. FYI, there are very few indoor track facilities in the DMV. Most schools use the PG Sports Complex. It is tough to get on the schedule there, so meets are infrequent. The Complex has open entry Sunday showcases in the winter, though.