How common is it for somebody to get osgood schlatter disease?

Depends: Males get osgood-shlatter's three to seven times more frequently than females. The incidence among athletes is ~ 21%, among non-athletes is ~4%.

Relatively common: Osgood schlatters is a traction injury of the patellar tendon (the one on the bottom of the kneecap) to the top of the shin bone. When the injury or inflammation occurs on the kneecap side it's called sinding larrssen johansen syndrome when it's on the shin side it's osgood schlatters disease. The treatment is rice (rest ice compression and elevation) occasionally an infra patellar strap is useful.