The rumor is at least two more, not including the already-announced LEGO Movie 2 and the LEGO Ninjago spinoff. Between this and the nine other Warner Bros./DC epics hitting between 2016 and 2020, it looks like the company is leaning hard on its franchises.
The studio has staked out release dates for two animated films: May 25, 2018 and May 24, 2019. These can't be The LEGO Movie 2, which already has a release date for May 26, 2017. And it can't be the spinoff LEGO Ninjago, which is slated for September 23, 2016. So these two dates are for two other movies.
Now, we're so far away from these days that things could always change. But if the upcoming LEGO movies do as well as the original, then it's likely that those dates are being reserved for more sequels. Whether it's two LEGO sequels or if one is for a prospective Ninjago is unclear. But Warner Bros. hasn't had success in the animated realm on this scale in a long time, so it makes sense that they want to capitalize on it.
On the other hand, it's always dangerous to rely on a franchise to build up a division. Maybe Warner Bros. is better off taking a cue from the surprise success of The LEGO Movie to invest in new animated projects. That seems like it's unlikely to be what's happening.