There can be 100 challengers. None of them will be viable against Trump. The entirety of the anti-Trump movement will coalesce around DeSantis, which Trump well knows. This is why he is attacking early and often.
The problem is that there are a sizable number of Republicans who aren't as conservative as Desantis, and may prefer more moderate Republicans in the primaries. People like Youngkin, or Haley. If the summer rolls around and there is no sign of DeSantis entering the race, that group of anti-Trump Republicans at least is going to rally around someone else with money, staff, etc..
The first GOP primary debates for the 2016 election were held on August 15th, 201
5. DeSantis would be giving up an awful lot of free airtime and exposure to other Republican candidates if he didn't enter the race until December. And if he did enter into December, after other candidates had invested so much time, money, and effort into the primaries, I doubt they would just drop out because DeSantis entered. And, DeSantis would have a much harder time hiring staff in those primary states because they already would have been hired by other campaigns. That's just a lot of catching up for him to do in a short period of time.