You can hold as many accounts as you like. The only limit is on funded capital: your total funded allocation cannot exceed $1,000,000 at any point.
Evaluation accounts: unlimited. There is no cap on the number of 1-Step Challenge evaluation accounts you can hold while you are still in the evaluation stage.
Funded accounts: capped by allocation. Across all funded accounts combined (1-Step and Instant), your total funded capital cannot exceed $1,000,000.