It's not from a single link, but just from a bunch of stuff I know. Feel free to link this post in your post or whatever if you want?
In Smash 4, there were empty "Character IDs" in the code that were found as soon as the game released. It turns out, these were meant for the DLC fighters. At launch, there were 9 character IDs in Smash 4's data. Later on, an extra one was added that was empty. By the time all of the DLC fighters released, every single one of those slots were filled. Bayonetta took up 2, Cloud took up 2, Corrin took up 2, and the other four characters each took one, for a total of 10. You can pretty obviously infer what a character ID is from that alone, but if you can't...
A character ID needs to be seperate if there's anything different at all in the programming. The most obvious example of this is characters with an alternate version for two genders, such as Corrin or Wii Fit or Robin, etc. This also includes alternate designs, such as Cloud's FF7/Advent Children skins, or Bayonetta's designs from both games. To give you a better idea, Bowser Jr. takes up EIGHT character IDs, one for each Koopaling. In Smash Ultimate, Hero obviously takes up 4, and Joker takes up 2. Piranha Plant DOES NOT take up 2, despite having a pot for some skins and a pipe for others. This is likely because the changes don't have any additional effects or real processing changes.
In Smash Ultimate, again, unused empty Character IDs were found in the data at launch. Exactly 29 of them. You may have heard it was 30, that was incorrect. There were EXACTLY 29. I counted them myself, in this case.
After Piranha Plant and Joker released, and they filled up as many of those slots as we expected, it can be inferred that the other 26 would be filled up too. We're now at 22, after Hero.
Seperately from the Character IDs, after Piranha Plant was released, they added a function to put DLC fighters into the Adventure Mode, World of Light. When Piranha Plant was put in this way, he brought 15 unused slots to do the exact same thing. These 15 slots are DIFFERENT from Character IDs, as they SPECIFICALLY MEAN CHARACTERS. They're used for unlocking the characters in WoL, so alt skins are not a factor here.
While there's no real precedent to say the 15 WoL slots are definitely going to be used, there IS a precedent to assume the 29 character IDs WILL be used, because they were in Smash 4. Smash 4 was made by the same programming team, and thus would likely follow similar structures and rules for how they do things. Since 29 character IDs is quite a lot, you could assume we'd get at least 10-15 characters just to keep everyone at an average of 1-3 character IDs each. This lines up really well with the WoL slots, of course.
Keep in mind, all of this is just information DIRECTLY DATAMINED FROM THE VIDEO GAME. Below is some speculation on what this likely means.
Piranha Plant was likely meant to be in the base game of Ultimate, judging by his Palutena's Guidance and some other things. This would mean the original plan was likely to have ALL DLC be part of a fighter pass, and thus multiples of five. We can pretty safely assume from this that more than one Fighter Pass had been planned since before Ultimate was released.
Because the WoL slots could mean echo fighters, and because planning out 15 full unique fighters around two years in advance seems ridiculous, it's very likely, at least in my opinion, that at least 5 of these WoL slots belong to echo fighters. My personal theory is that we're going to get a second fighter pass that has echo fighters bundled in, instead of stages.
That's about all there is to say on it, I guess.