I think it notifies you for each like separately, I could be wrong though. In any case, if you look at the post it will say " 3 people like this" or similar. Click on that and you’ll see the avatars for whoever liked it. If you’re not sure who’s who, click the picture and you’ll get a user card.
EDIT: Just noticed, if you are LOOKING at the post, the notification will mark itself as read (it’s still there though). So, I see you just liked this post, and I did get a notification but it didn’t make a blue dot. That’s because I’m still looking at this post, and I saw the like first hand - it assumes you don’t need to be notified when you already saw it.