Not the first and not the last about a thread being make on this topic. Yes there a few people who insisted about two but i am very unsure why.
General rule it is 1 per player per day on the island. Apparently there are some outliers but nothing to stress over. Also you can get extras on mystery islands.
So based on my observations from tting and multiple characters. I believe the people claiming two bottles on one character is because they either logged in with one of their extra characters AFTER picking up the diy bottle with the main character and then did not pick up the newly spawned bottle from the extra character. So when they go back to the main character, they find another bottle! (This also happens if youre playing local coop by “summoning” Another character(s) with an extra set of joycons.
I can tell you that I’ve only gotten 2 bottles when I TT forward and then back to the current date, before 5am but after 12am, so it’s technically the “same day”, but the game doesn’t really roll over until 5am.
I've only ever gotten one bottle per day. Regular Switch, two users, only one user on AC, a few friends on my friends list. Green airport. I completely close the game at least once daily
Edit: I've recently stopped picking up the bottle, and it's always just one in the same place every day
I’ve gotten 2 in a day on multiple occasions. I had this discussion with a friend. They believe, or read that if you pick up the bottle early morning, you’ll get a second one. I often play at the start of the morning, 5am. I’ll pay more attention to this and see what I can figure out. I only have 1 character.
Only time I've gotten two in "one day" is when I played before 5am. Picked up the bottle, got Isabelle's announcement, found another bottle. I play every day so there's no way the first bottle was the day before's.
I get 2 per day if I go on in the morning and again in the evening. I rarely play in the mornings anymore though so I end up only getting 1 per day. I have multiple people on my Island and they don’t play every day. When they do they always get their bottles. It’s been this way since I started playing in April and we don’t time travel too often because we don’t have a reason to. I never want to mess up someone’s turnips either. However, there have been days where I haven’t gotten any at all. This game is wonky.