I was so used to Trek books every month when John Ordover was editor.
The standard is still one mass-market paperback per month. That only falters in unusual circumstances, like when the four planned Abramsverse novels were pulled and left a gap in the schedule.
It was shock to me after Marco was no longer editor.
It was a shock to all of us.
I liked the DS9 arc of books each month and then you see them streched out to the point where it's a yearly thing or every six months or more. If there was a good Star Trek show on the air I believe there would be more interest inthe novels.
With so many different series, it was fairly common for any given series to get only 1-2 books per year, or even less, depending on the year. Some years one series would get more focus due to a particular event, and then the following year that series might not be heard from at all because another series was the focus of the year's big event.