How many games is he going to play? How many times will Pep bench him for no reason? How fast is he going to adapt? There are a lot of factors in play. 20+ goals is possible, but not as easy as some may think.
Haaland scored more than a goal per whole game played for Dortmund in all competitions.
Furthermore he has so far scored 23 goals in 1463 minutes in Champions League and has only taken two penalties. That means 1 goal every
63rd minute.
Compare that to:
Messi 1 goal in every 104 th minute
Lewandowski 1 goal in every 105 th minute
CR 1 goal in every 114 th minute
Mbappè 1 goal in every 125 th minute
Aguero 1 goal in every 126 th minute
Salah 1 goal in every 157 th minute
And all of those players have played most of their games in huge dominating clubs while Haaland has been playing his games in clubs like Salzburg and Dortmund, and let's not forget, while being under 22 years of age.
Haaland isn't the best player of the lot but he is the best goal scorer the world has ever seen (with the possible exception of Gerd Müller). He is definitely capable of scoring 50+ goals only in PL if he plays every minute for a team like City.
I have made a bet for Haaland scoring 35+ goals in PL alone and that is practically money in the bank if he plays more than 30 games and takes penalties (which he should considering what we have seen of City in the last two years).
No one buys a striker with his record, pays that transfer money and wage and think that it is not going to be easy for him to score more than 20 goals. If Haaland scores less than 20 goals that would be a massive fiasco.