What is the limitation of responses via a starters plan for a form ? it says 100 but is that per year or per month ?

As a free (Starter) user you can receive up to 100 submissions per month. This limit is for account not per form. For example if you have 2 forms on your account and one of them receive 80 submissions and other receive 20 submissions, you will reach your monthly limit of 100 submissions per account. Submission counter is reset at first day of each month.

More about different account types you can find on our pricing page. 

Hope this will help. Let us know if you need further assistance.