Put in your account size, the premium and how much you'll risk. It tells you how many contracts fit. It assumes the worst case, because a bought option can go to zero.
About the 5–10% cap: 360 Trader teaches risking no more than 5–10% of your account per trade as a maximum if you start with a smaller account. It's a ceiling, not a goal. Plenty of traders risk a lot less. One option can lose 100%. At 10% per trade, two total losses in a row would cost about 20% of the account.
Contract cost = premium × 100 shares. The result rounds down and excludes commissions and fees. This is an educational tool, not a recommendation to trade any security.