I have 10 novels in progress on my desktop right now. I'm writing the second in my gaslamp series (which is definitely going to hit three, and maybe four.)
After that I've already got the first 10k or so for book 11 in my longest-running series. I just don't seem to run out of ideas with that one, and the characters are like a comfy pair of slippers. By the end of each novel they're smouldering wrecks, mind you.
Then I've promised myself book three in my 'serious' space opera series. There's a ton of combat in those, which is very quick to write and just seems to flow onto the page. I doubt I'll write any more past book three.
After that it's book four in my female scifi cop series, with the rough plot and chapter 1 ready. Again, book four will probably be the last.
Book 12 in the Hal series, with 13 through 20 on the drawing board, lol. (They tend to 80k each, not exactly short!)
Finally, I'm planning a second trilogy in my comedy fantasy series (Dragon and Chips books 4-6)
If I write at this year's rate, excluding this week, that lot will take me until 2023. If I write at 2018 speed, they'll be done by November next year. If I write at the same pace I've managed these past two days, I could write 800,000 (all ten novels) in five months. And then have a nice memorial service for my wrists and fingers.