Most (if not all) will depending on where you are in the process. If booking a return or multi-segment and building an itinerary page by page, the second you select a first segment and move to the next page, a PNR will be created with that segment in it taking the seat out of inventory. If all the options are on the first page and you click next to see the full price, a PNR will be created with those segments in it taking the seats out of inventory.

In order to price and book, the PNR has to be already created with the requested segments in it, which means the seats deduct from the available count. When you close the webpage without booking, the PNR doesn't often immediately drop, but rather stays in the system for some amount of time. If an airline or travel agent was making the booking directly in the GDS, they could cancel the PNR and immediately return the seats to inventory, but that isn't what happens when you close an airline website in your browser.