You can do it in one, although we like a second day. One day is fine if you don’t do the water park.
We also really like to stay at the Legoland hotel - it’s half of the experience IMO - and staying at the hotel usually makes sense for us to do at least two park days. The park hours are pretty short.
We just did our 3rd trip to LL in July and bought annual passes when they were on Labor Day special. Planning to visit a few times this year. Happy to answer any questions.