if i remember correctly, LOR can't be uploaded onto ERAS until you have certified your applications anyways. I certified my ap and applied to programs as soon as I had finished it, mid sept if I remember right. and then waited for my LOR to roll in. i think i had 2 LOR in by early Sept, a 3rd by the end of sept, and my 4th in mid Oct.

this whole process is crazy and there is no correct answer. some programs won't look at your ap until all LOR are in, others will look at it earlier.

If I remember correctly, you can submit your application usually around the first week of september and add your LoR's as you get them. Most programs will start downloading your ERAS materials middle to late September. I know a lot of programs will download your applications multiple times throughout September and October. I know I had made a few additions to my application and had LoR's added later in the season which got downloaded multiple times. And, you can follow along to see which programs have downloaded your application materials (and the dates it happened) and which ones have not. So, you could actually follow along with that. Personally, I would recommend getting in as much as you can as early as you can. But, that's just my humble opinion. Good Luck!