We don’t know. We will have at least 3 seasons for sure: the current one, season 2 in 9.1, and we will have at least a third for the final patch of Shadowlands. There may be other seasons inbetween, or maybe there will only be three.
All we know right now is that we’ll have at least 3.
BfA and Legion had 4 seasons tho, if that’s any indication.
If Legion and BFA are any indication, there sould be 4. However, if you consider that Shadowlands resembles WoD the most (unskippable intro quest, the setting being isolated from Azeroth in space and time, bad storytelling, long dailies to fill bars, raid logging possible, but most importantly: first content patch coming very late), then it’s probably 3.