If a person is on probation and is caught with 20 dollars worth of drug and 20 dollars under the passanger seat should he pled not quilty if its not his how long most likely will he get if he does plea quilty

I would assume that any violation of the probation agreement would result in this person being given the amount of time that was agreed in the terms of the probabtion from the original offense. Most likely that would be whether you are charged, guilty or not. Suggest for them to be proactive and honest with their probation officer.

In most states, it doesn't matter if it is theirs or not because the legal ruling is it is your car
You should know what is in your car so it is your problem. SOME will drop the charge if someone comes forward and says it was theirs and you knew nothing about it. Strange but I HAVE known that to happen.
It would depend on his charges, priors and so forth how long he would get.