Well, it depends on how many icons you need.
If you need icons that cost less than $9
The minimum price of an icon is $2. If the total price of the icons that you need is less than $9, then buying icons one by one is the most cost-effective choice. For this, you can add the icons to the shopping cart and pay in bulk or make a prepayment to your account balance and download the icons in one click. Read here about how to make a prepayment to your account balance.
If you need icons that cost more than $9
In this other case, choosing to subscribe to either Pro Micro or Pro Starter plans for 1 month might be better for you. With Pro Micro plan, you get a total of 10 icons for $9/month. Pro Starter subscription plan gives you 50 icons for $19/month.
If you need A LOT of icons
Then, the Pro Unlimited subscription might be the best option. You can download as many icons as you need for $49/month.
Note: if you are not planning to use the subscription for longer than 1 month, you'll need to actively cancel it before the billing period is due, as you will be automatically billed for the following months, otherwise. You can cancel your subscription anytime from Your account page. Read here about How to cancel your subscription.