According to rumors, Super Smash Bros. Ultimate might have 16 DLC characters planned for the future.
Super Smash Bros. Ultimate will have at least six DLC characters throughout its run. Piranha Plant is available right now, Joker will be released sometime before the end of April, and the remaining four are due to be released sometime in the future.
There is a lot of speculation that Ultimate will have more DLC characters in the future and a recent find within the files of the game may point to that, as there may be a total of sixteen DLC characters planned.
A modder in the Super Smash Bros. community named Mizumi has claimed that there are sixteen dummy slots within the World of Light mode in Ultimate and that one of them is taken up by Piranha Plant.
It bears mentioning that you start out with Kibry in the World of Light mode and have to unlock the other characters, but Piranha Plant will be made available straight away if you have redeemed the code/purchased it, which suggests that the same will be true for the other DLC characters in the future.
It's possible that Nintendo was just hedging its bets and leaving more slots open than it would ever need in the World of Light mode, but it makes a lot of sense for there to be more DLC planned for Ultimate outside of the Fighters Pass.
Super Smash Bros. Ultimate was released at an early point in the lifespan of the Nintendo Switch, and the next game in the series will probably be saved for the Switch's successor. The Switch's success means that we won't see a Super Switch/Switch 2 for years to come, which suggests that Super Smash Bros. fans will be playing Ultimate for a long time.
The first Fighters Pass has five characters planned and Pirahna Plant was used as a free character for early adopters, which accounts for six characters. It's possible that we will see two more Fighters Passes released over the next few years, each with five characters of their own.
The first Fighters Pass costs $24.99, so the release of two more of them would allow Nintendo to earn a lot more money from Ultimate over the next few years and could bring the game in line with a lot of the live service games that other developers have been making.
A Super Smash Bros. game that is continuously receiving updates throughout the lifespan of the Nintendo Switch would likely be a huge money spinner for Nintendo, so it's easy to believe that sixteen DLC characters could make their way into Super Smash Bros. Ultimate in the future.