https://www.quora.com/How-does-Google-Code-Jam-work
There is no limit on how many times one can participate in Google Code Jam. However, there are some terms and condition which you can find on the website itself however I am posting an extract here.
1. Eligibility.
- 1.1 VOID WHERE PROHIBITED. The Contest is void in Crimea, Cuba, North Korea, Sudan, Syria, Quebec, and where prohibited by law.
- 1.2 NO PURCHASE NECESSARY TO ENTER OR WIN. You do not need to purchase any Google product or service to enter or win the Contest.
- 1.3 Ineligible Individuals. (A) You cannot participate in the Contest if you are, and will be immediately disqualified and forfeit all of your prizes if you become: (1) a resident of Crimea, Cuba, North Korea, Sudan, Syria, or Quebec or anywhere that the Contest is prohibited by law; (2) restricted by applicable export controls and sanctions programs; (3) a current employee (including intern), contractor, officer, or director, of Google or its affiliates; or (4) an immediate family member (including a parent, sibling, child, spouse, or life partner regardless of where you live) of one of the individuals listed in subsection (3) above or you are a member of their household (whether related or not). (B) However, Section 1.3(A)(3) above does not apply if you are a Google Student Ambassador (whether paid or not) and you may participate in the Contest. (C) If you gained information on a problem while working as an employee, intern, contractor, or official office-holder of Google, you will be disqualified from the contest if you attempt to gain points on that problem.
Winning prize or going onsite is a lot different thing. So, you can participate how many times you want. Good Luck!