Unless they change their minds, it’ll be 3 expansions per year. It used to be 1-2 expansions, and the rest were Adventures, but Blizz decided last year to only do expansions from then on. So every 4 months expect a new expansion.0
Well, at first they had an adventure and an expansion. Then they did adventure - expansion - adventure for one year and that was thought to be the final model. Then there was expansion - adventure - expansion, following the previous model, but it was stated that all years would now be two expansions and one adventure. That lasted one year. Then we got the three expansions model that has now lasted for over a year.
It's been said that the model of three expansions with single player content each year will stay, but the same was said about all the other previous models so I wouldn't make any assumptions.