How can I know which plan will be good for my case?

I am creating a form I am going to send out weekly to 100 people (approx), not all of them will fill them out.
Do I have to go to the next tier? 
I think I can use the same form every week, but I might have to have a unique form every week.
What are my options...really want to use it for free for a while to see if it works for my situation

It would be a bit hard to know how many submissions you would be receiving per month, note that the limits are counted in a calendar month. I would recommend you to start working with the free account and you may upgrade if needed, note that you may upgrade/downgrade at any time.

This guide will also help you checking your account limits and usage: https://www.jotform.com/help/408-Understanding-Your-Account-Usage-and-Limits 

I think I can use the same form every week, but I might have to have a unique form every week.

I did not understand this par; however, note that forms can be re-used if you want, but all the submissions are collected in a single place and you can check them following this guide: https://www.jotform.com/help/269-How-to-View-Form-Submissions 

You may also download the submissions data, here is a guide that will help you doing so: https://www.jotform.com/help/73-How-to-Download-Form-Submissions-as-Excel-CSV-PDF 

I hope this helps. 

Is there a limit to how many forms you could have an a month
Sent from my iPhone
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The number of forms each account type can create does not reset monthly, it is fixed. Please check our Pricing page to find more about different account types and limits. Also, please check the following guide:

Understanding Your Account Usage and Limits