Well, it depends on how many icons you need.
If you need 9 icons or less
Usually, the minimum price of an icon is $1. There might be the case that the icons you need cost less than $9. In this case, buying icons one by one is the most cost-effective choice. For this, you will need to make a prepayment to your account balance. Once you have a positive balance, you can start buying the icons you need. Read here about how to make a prepayment to your account balance.
If you need 10 icons or more
In this other case, choosing to subscribe to Pro Starter for 1 month might be better for you, since you get a total of 25 icons for $9.
As a note, if you are not planning to use the subcription for longer than 1 month, you'll need to actively cancel the subscription before the billing period is due, as you will be automatically billed for the following months, otherwise. You can cancel your subscription anytime here. Read here about How to cancel your subscription.