Quantity

When you select a strategy from the dropdown list, you choose a strategy template with a minimum number of contracts in each leg. Consider the following examples.

The vertical spread contains two legs with an equal number of contracts, so the template assumes that each leg has 1 contract.

Asymmetric strategies may have various numbers of contracts in legs. For example, the strip strategy assumes that you buy twice as many puts than calls. Thus template has 2 puts and 1 call.

The quantity selector is just a multiplier for sizes given for each leg.