Only one, and if you installed it on your parent's system as well you would be violating the license agreement in two ways.
1: They most likely do not qualify for the reduced price student edition
2: You bought a SINGLE license, which means you can install and use it on a SINGLE system at any given time
If you wanted to install it ONLY on the system at your parent's place, that would be fine so long as you were there and using the system, but when you went back to school your parents would not be licensed to use that copy of Windows.
They're going to have to shell out the $200 for a copy of their own or make due with what they have. That's just how it is. If you install it on both systems, you'll likely find one won't activate, and Microsoft could revoke your license at their discretion.
So long story short: Don't do it.
I bought Windows 7 home edition at the student price. My parents want me to put Windows 7 on the home computer also. I was wondering how many users I can use the key for. If i can only use the key for one user, do you think I can purchase another key from Microsoft at a discounted price?