There is no limit to the number of Evaluation Challenges you can purchase or have active at the same time. You may have multiple PRO, FLEX, RAPID, and AUFX evaluation accounts running simultaneously.
The limit applies only to your simulated funded capital.
Each trader is allowed a maximum of $100,000 in active simulated funded capital per user/name at any given time. This limit applies across all Aurex programs combined, not separately to each program.
For example, even if you already have a passed and active funded account, you may continue purchasing and trading as many evaluation-phase challenge accounts as you want. You are not violating the $100,000 cap simply by having additional challenges.
The restriction only becomes relevant when those challenges are passed and would cause your total active simulated funded capital to exceed $100,000.
In short: unlimited Evaluation Challenges, but a maximum of $100,000 in active simulated funded capital per trader.