@Mojoisme1 50 chapter is a common milestone for anime adaptation. At that many chapter, the studio can get enough material to be selective on their adaptation (lets be honest, the weekly release of manga tend to be "milked" to keep releasing chapters/story every week, and not every chapter is good. m
Meaning it is quantity centered, and not quality. Not every chapter is a masterpiece & has good enough core idea to be adapted on screen. At this, monthly manga has better material to be almost 100% adapted. For weekly manga, even 70% is a lot. Many studio tend to be selective and cut the "filler" chapter in weekly manga adaptation)
At 50 chapter, the manga has atleast 2 seasons worth of episodes (if it adapted almost 100%, which is almost impossible), but at most will be adapted for 1 season + some OVA (remember, the studio need to cut the filler chapter). At that many chapter, the manga will continue to release with anime on air, giving it a major boost in sales, and at some point can ensure the continuation of the series. Giving it more material for the second season (cause the first season not adapting all 50 chapters, it give more material to be added, and second season can release before the hype is totally diminished, usually in half-to-one and a half year).
Less than that it will have a risk of, not only a draught of material but a major risk of not earning enough boost (probability of being axed), author's consistency of the quality of storyline, and diminished hype for the next season. If you release the anime in 30's chapter, you need to wait for another year to have a complete material for the second season, and it by the assumption that the story has constant good enough quality, which at many occasions of weekly manga is impossible. The fans need to wait more than 1 year for the second season, at that point, the hype will be less the longer the wait. 2 years? Some will wait, some will forget. 3 years? It's better to make anime for other manga. The fans is there, but the hype will be almost diminished.
Edit: i forgot to say this, "the more the material the better". Even 50 chapters is the milestone, it is still considered minimal. The ideal is 100 chapters. It will give the studio so much material and at the same time more insight to plan for the next season, what to cut and what to stretch. Where to start the season's episode and where the ideal end/climax.