IF one counts the "babies" on the spider plants, probably over 100. If not, then it's around 15 - 20. What's the one plant that you have?

Does a conservatory count? I probably only have about 6 in the house itself but the conservatory is a topical jungle.

I lost count a couple years ago.
I started out with houseplants, then to orchids.
I discovered our local grocery store was throwing out orchids out of bloom-nothing else wrong with them.
Their "unmarketability" was my boon. I have Vanda that I paid no more than $5.00 for!
My condo filled up quickly.
Unfortunately most are NOIDS and cannot be shown or pedigreed

I have none because our flats are so hot they wouldnt survive but we have some in communal areas and I have 3 cacti on the upstairs windowsill.

Your question made me stop and think,
none,
did a recount, still none, which actually surprised me, but have never thought about having any in this house, that said we have a mini forest outside every window, have to keep the summer heat out somehow.

Cats and houseplants don't mix .

Currently 48. Will have to bring in more as the nights get colder.

Here are a few that I recently installed in a spot that requires a ladder to access:

6 in my apartment right now, but I used to have more before cats. The little jerks find them quite tasty. The 2 cats I have now are what you might call "floor cats," they really don't jump up on anything, so I'm at least able to keep plants in the window sill again.

I have zero. 3 dozen rose bushes in the garden, 3 hydrangeas, a couple loropetalums, one mandevilla vine, as well as plans for some Clematis and Lantana and herbs in the spring. Plants indoors seem just terribly anti-climactic. The light is disappointing, and seasonal changes are next to non-existent. Just not as interesting to me. I do plan to set up a proper propagation shelf, with lamp and north-facing window and all. But those aren't houseplants.
Oh wait! I do have a striped spider plant, technically a houseplant, but mine in growing outdoors, on the edge of my carport.

Some where around 25...

Zero. Our two cats are "gardners".
In the winter, I set up grow lights over the bathtub in one of our bathrooms and start plants in there for the spring planting.

I used to have many, many, many. Then I started moving to apartments, and apartments didn't have good natural light. Then I move to house that doesn't have good natural light. And then I got KITTENS! Now I have maybe 7. Mostly where cats can't get them and also where I forget to water regularly.

The only plants I have inside are my cuttings when I’m trying to get them to root. And then I have to protect them from the cats

Five in my house right now, but more when the cold weather sets in.