You can purchase unlimited challenges. However, you can hold a maximum of 10 active Funded accounts.
Within those 10 Funded accounts:
You may have up to 5 funded accounts from Elite, Custom, Instant, and Nitro challenges combined.
You may have up to 10 funded accounts from Prime challenges.
Nitro challenges are limited to a maximum of 3 accounts.
These limits apply to funded accounts only; you may continue purchasing additional challenges as needed.