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 also use Compare Attribution Settings in Ads Manager to break down how many of your conversions 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).