The maximum number of contracts you can open varies for each Challenge.
Futures Classic
$20,000 Challenge: 10 micro contracts.
$150,000 Challenge: 5 standard/mini contracts or 50 micros. You can also combine standard/mini and micro contracts. For example, 3 standard contracts and 20 micros.
$200,000 Challenge: 7 standard/mini contracts or 70 micros. You can also combine standard/mini and micro contracts. For example, 4 standard contracts and 30 micros.
$250,000 Challenges: 10 standard/mini contracts or 100 micros. You can also combine standard/mini & micro contracts. For example, 7 standard contracts and 30 micros.
Futures Prime
$50,000 Challenge: 5 standard/mini contracts or 50 micros.
$100,000 Challenge: 10 standard/mini contracts or 100 micros.
$150,000 Challenge: 15 standard/mini contracts or 150 micros.
*Keep in mind, the lower your current balance is, the fewer contracts you will be allowed to trade.