Option 1: Next to the block with the coupon code - there is a block with the maximum redeems. In this block you can state how often the code can be redeemed. If you set it to 100, then only the first 100 people who redeem the coupon can use it. The rest will be unable to use the coupon.
But this would be for a private sale, for example, one you host on social media. It's not shown to itch.io users.
Option 2: You can generate say 100 keys and send them out to people. Only those with a key can play the game. In this case you can set a requirement that the key must be claimed- the person must claim the game via an itch.io account - which should technically prevent them from sharing the key (they might still share the download link). This is manual - you need to send everyone a key, or use automated software like mail merge. But is useful if you send out newsletters, use Patreon, or participate in a give-away site.
Option 3: You can put in the description that the sale is limited to the first 100 people who claim it - and then end the sale immediately after that happens. This requires you to watch the sale. (I guess, this would be your request for itch.io to create a script that would automatically limit it - but it doesn't necessarily require a coupon.)
But keep in mind, Itch.io doesn't do DRM, so limiting anything is difficult.
