There is no limit on the number of challenges you can purchase simultaneously. However, the maximum total funded capital you can hold across all your funded (simulated live) accounts is $300,000.
For example, you could have three funded accounts of $100,000 each, or one of $200,000 and one of $100,000 — as long as the combined balance does not exceed $300,000.
If you breach a funded account, the corresponding allocation becomes available again for a new challenge. All accounts must be registered under the same profile — creating multiple profiles is prohibited.