Here’s how ads get credit for a conversion…
By default, an ad gets credit if someone converts within 7 days of clicking your ad or 1 day of viewing your ad without clicking. This is a view-through conversion.
You can change the attribution setting in your An ad set is a Facebook ads grouping where settings like targeting, scheduling, optimization, and placement are determined. More. You have options of 7-day click or 1-day click, 1-day view or no view-through, and 1-day engaged view for video ads.
You can also use Compare Attribution Settings is a feature within Ads Manager reporting that allows you to view how many conversions happened within each attribution window, regardless of the Attribution Setting used for optimization. For example, columns can be added to your report for 28-day click, 7-day click, 1-day click, and 1-day view to see how conversions are distributed across them. More in Ads Manager to break down how many of your A conversion is counted whenever a website visitor performs an action that fires a standard event, custom event, or custom conversion. Examples of conversions include purchases, leads, content views, add to cart, and registrations. More happened within each of these windows.
Finally, to address some confusion… only one ad gets credit for a single conversion. In the case of someone seeing more than one of your ads before converting, the credit will go to the most recent click. If there was no click, it goes to the ad with the most recent view (assuming it happened within a day of converting).