after they will register I want to send them back the number of their group. And there can be only 20 person in each group. Is this possible?

Sorry for not getting your requirement clearly.

So far, I presume, You want to send a number of a group to the users upon their registration.

You can send the group number as a hidden value to the users in their autoresponder email.

So once they fill out/register the form, they will get the group number in their autoresponder email.

You can add a hidden field form using the Widget panel on the left of your form builder and then select hidden box.

Follow this link to know how to setup autoresponder email: https://www.jotform.com/help/26-Setting-up-an-Autoresponder-Email

For any further query, please let me know.