If I expect to receive payments embedded in a form from well over 100 people in one month, which plan is needed?

If I expect to receive payments (via Paypal or Square) embedded in a form from well over 100 people in one month, which plan is needed?

If you're expecting 100 payment submissions per month, I recommend the Silver Plan. If you're expecting more than 100 payment submissions per month, the best choice would be the Gold plan which has unlimited payment submissions. For the pricing, please check this link: https://www.jotform.com/pricing

To upgrade your account, please follow this guide: https://www.jotform.com/help/331-How-to-Upgrade-My-Account

You can also learn more about account usage and limits on this link: https://www.jotform.com/help/408-Understanding-Your-Account-Usage-and-Limits

Please contact us again if you have questions.

If I select a lesser plan but notice more submissions are being received
than expected, will I be able to upgrade at that time in the middle of a
month?
...

Yes. You may upgrade the account as per your requirement. However, please note that you'll be able to upgrade the account to higher plans. For example, if you upgrade your account to Silver now and received 100 payment submissions, you may the upgrade to Gold subscription plan then as you needed. 

Hope this information helps!