I only bought 200 turnips for my first time. I haven't been to any other islands yet (other than the in game ones using Nook Miles Tickets), so figured I wouldn't waste too many bells on them before I've done that a few times.I always tend to get an inventory’s worth and no more in case I need to go to someone else’s island to sell. Not doing two trips.
I'll try and keep it a casual side thing rather than spend too much time on them I think.