Uhhhh... a lot. Big ramble here.
If we're talking just Nijigasaki, there are three albums. The first two contain 10 songs each and the third has 12 for a total of 32.
Then there are 3 singles with 2 songs each, that makes 38.
11 insert songs in the anime make 49.
Then there are sub-units, which are smaller groups who sing together inside the main group. Nijigasaki (currently) has 3 of them. They've all done three sets of one song each bringing up the total up to 58. Then each of them are in the process of releasing short versions of their 4th songs which are all themed around a specific concept. DiverDiva's is a summer shrine visit, AZUNA's is for Cat Day, and QU4RTZ has one for Christmas that hasn't been released yet. So that's 60 with QU4RTZ's song on the way to make 61.
As for the other groups, μ's is known for having a lot of songs, totaling at least 120 for the 4 years they were active for, which includes their albums, singles, insert songs, duo and trio songs, sub-units, all that. But there are different solo mixes too. For example, Snow Halation is a μ's song sung by all the members, but there are different solo versions for each member specifically. This means that technically these group songs have 10 versions each: One for the group and nine for solo mixes for all the members.
Aqours has a lot as well, probably at least 100 if I had to guess? Aqours just didn't get as many solo mixes as μ's did until now, where they're releasing solo albums for each member on their birthday. Aqours will probably hit the same amount as μ's has and likely go even further.
There's a fourth main group, Liella, but they're pretty new and only started making songs this year, so they've only got about four.
Then there's several smaller groups or songs that just don't fit in anywhere. Songs for characters in the rival groups, fit in here for example, so there'd be about 15 of those.
So yeah. Short answer, there's a lot.
Nator?