It'd be possible that a person with cerebral palsy start to walk on their own if they pratice alot?

Depends on severity: This is a broad question since so much depends on the cerebral palsy, which muscles are involved, the degree of involvement, and the state of health of the joints. People affected will function to the best of their ability, and will eventually figure out a way to move around, but that "practice" is often guided by physical therapy to learn best ambulating techniques.

Possibly: CP is a broad collection of movement disorders. Each case is unique & outcomes are variable. Early and agressive therapy and medical management of associated problems can provide a chance of upright mobility later. At some point the affected kid may let go of trying to move like others & decide s/he is too slow that way & a wheel chair or other assistive device makes life better.