Whatever you are capable of handling in a way that doesn't affect your school performance, your performances for your teams, and your personal health. I'd have gladly double-teamed when I could have (didn't realize I was a 2-year super senior :( )
Oh, and I disagree with those who say 'X' sport isn't year round. For those who want to make something major out of it, it's year-round. When my brother isn't doing HS volleyball, he's doing AAU/allstar, playing on the sand courts in the summer, or even helping out my mum coach JV at my old HS. Or if you're Faith Martin (she just graduated from my HS- her mom was an Olympian and you can bet she'll be one too! She's an unbelievable swimmer and could have basically picked where she wanted to go to school), you're in the water every day, all the time. The girls who play basketball at my HS are constantly playing, either school, AAU, summer ball, fall workouts or a cross-training sport (like volleyball). I can name at least 3-4 other kids I know who played other sports who didn't stop just 'cause their season was over..even if it just meant team workouts or doing a little free play (very similar to our summer practices). Heck, some played 2 different sports at once!