The number of customers you can sign up is limited by your seat quota — the seat depends on the partner plan you have contracted (Manual 39, Billing), it isn’t unlimited by default. If you try to sign up a customer with no free seats, the form prevents it with an honest notice instead of creating the tenant:
Note
A newly signed-up partner, before choosing a seat plan, starts at 0/0 seats: the first step is always choosing a plan in Billing (Manual 39) before you can sign up your first customer.