I think it will be neither 400 or so nor 50, I bet on 150 give or take.
If the statement about 2024 as a deadline holds any slight truth, then this manga is not making it to 1400 chapters imo. Rather 1300 at best, and even then, that would end later.
50 chapters for Elbaf and around 150 for the final war, maybe 100 for the Laugh Tale journey + endings. That would be around 300 already.
It's too soon to make predictions anyway.
Again: Oda said that "the most exciting part of the story will be over in 5 years aka 2025".
It's pretty obvious that he means Luffy will become PK by then, since that's what Oda deems as most adventurous/exciting part of the story.
However, it's also clear that Oda emphasizes the fact that the manga itself will go on after that.
Right now we get about 35-38 chapters per year. 5 years would mean roughly 175-190 chapters until Luffy becomes PK. Since Oda never holds his deadlines, let's round it up to ~200 chapters.
That seems reasonable. About 70 chapters for Wano to finish (5 more for act three, 50 for act four, 10 for act five, 5 for in-between chapters), 70 chapters for Elbaf and 60 chapters for Laugh Tale and the final fight against Blackbeard.
After all that, the final war itself needs to be prepared of course and the war itself will dwarf Marineford, Oda said so much.
The entire Marineford saga (Impel Down + Marineford) lasted around 60 chapters, so it's reasonable to expect something like 80-100 chapters for the grand finale.
All in all that means we have roughly 300 chapters left, which means around 8 years in real time: 5 years for Luffy's PK adventure to conclude, 3 years for the final war saga.