Can I set up a limit of registration for questions? Is there a way to have payment required to complete registration form?

Can I set up a limit of registration for questions?

I am not sure if I understand your query correctly. Do you mean to limit the form submissions to a number? If so, it is possible by setting up the option 'Disable on submission limit' in the form settings. Please refer to the guide below:

https://www.jotform.com/help/137-How-to-Limit-Submissions-in-a-Form

If you are referring to something different, please let us know.

Is there a way to have payment required to complete registration form?

If you want to receive payments to your forms, you may consider adding one of the payment methods to your form. Please refer to the following guides that can help you with it.

https://www.jotform.com/help/165-Setting-Up-Your-First-Order-Form

https://www.jotform.com/help/323-Mastering-Payment-Form-Integrations-with-JotForm

Hope this information helps! Please get back to us if you need any further assistance. We will be happy to help. 

Thanks! Though, I meant something slightly different. 

How do I set a limit of respondents for answer choices? For example, if there are only 50 seats in a workshop, I want to cap the respondents to 50. 

Example below...

Question:

"Choose your Medical Lab Choice:"

Dropdown Answer Option Choices:

Choice #1: Trauma (Limit to 50 respondents)

Choice #2: Prevention (Limit to 50 respondents)

You may use the Inventory widget for that, here is the link: https://widgets.jotform.com/widget/inventory 

This guide will help you adding it to your form: https://www.jotform.com/help/252-How-to-Add-a-Widget-to-Your-Form 

You will need to perform calculations and then pass the result to the payment field, please take a look to the following guides in order to get more details about the calculations: 

https://www.jotform.com/help/259-How-to-Perform-Form-Calculation-Using-a-Widget 

https://www.jotform.com/help/275-How-to-Pass-a-Calculation-to-a-Payment-Field 

I hope this helps.