I am building this for a road race, If more than 100 people fill out the form in a month it will cutoff?

The way I just read the information is that if more than 100 people fill out the form I must pay to extend that?

You currently have a FREE account. This plan allows you to receive a max 100 submission. This includes all your forms you have created. Once you have received 100 submitted forms in total, you forms will be disabled. 

The limit counter reset the 1st of every month.

If you have any other question, please let us know. We will be glad to help.

So I can upgrade for a month and then cancel? I only need this for race registration it would be really silly to keep it annually

Yes, you can upgrade and cancel your subscription any time you like. If you are having difficulty upgrade your current plan or downgrading, please let us know and we will be glad to assist.