I don't think that's correct. I've been playing since launch, multiple times a day and always run by the beach several times over to collect shells.Confirmed by myself and a friend you can get up to 3 per day per character, mount of characters on your island makes no difference in how many you get on a single characters save. So I have 4 characters and can get up to 12 bottles per day if I check back in morning, noon, and evening. It doesn’t seem to be guaranteed though, and seems to have a higher chance of spawning if you’ve saved an quit, then reloaded in a few hours later to play again!
I always get one bottle every single time.
I did read that having multiple characters increases your chances of getting additional bottles but haven't actually tested it out yet.