Is there a limited number of collaborators on the project? In my project I count on working with three Czech partners.

In this case, these are national level collaborators who will be listed in the Application Form as co-applicants, because you as Czech partner / lead applicant, you will share part of the national funds (as you plan) with the collaborators. However, they will not be listed in the pre-proposal or full proposal, only you as the Czech (lead) partner will be listed there.