No there is no such limit Exist. you get only one attempt to clear exam and if you failed then you have to purchase adwords Exam once again. And google will be happy that you are purchasing same exam many time increasing google revenue.
Name :-Harsh Pareek [ This guy guarantee 90+% mark in all adwords examination [ fundamental, Advance and display ] at affordable prices. His email-id is "pharsh88@gmail dot com" and contact no:- +91 9892854892. First time when i had given examination he helped me. I would suggest to try him.
Jayesh there is no such official SEO exam by google exist. yes but with Adwords examination me or harsh both can help you. There are total 3 exams for PPC [ fundamental, Advance Search and Advance display ]. You need to buy these exams from google directly. Otherwise inbox me at "" or call me at +91-9146718386 for further assistance. Harsh contact information is already provided in my previous reply to woits. Also you can refer "http://www.google.co.in/intl/en/adwords/professionals/" to know more.
Official Google Certification Program Help Center where you can find tips and tutorials on using Google Certification Program and other answers. visit here-https://support.google.com/adwords/certification/answer/12252?hl=en find all the question about adwords.