I only do new Grammar items when I ‘feel’ like I’ve got a handle on my Beginner and Adept level grammars. Usually this means that they are at a ‘low’ count. Currently that means that Beginners are less than about 15-20. (Also, my Ghosts should be equally ‘under control’, like less than 20. I have Ghosts on the default setting, so I tend to get a lot of them over time.)
When I do take on new Grammars, I do three at a time.
But, realistically, there can be times when there are many many days between new Grammar lessons. So, actually, it’s less than even 1 per day, to be honest.
I’m also in the middle of doing a huge number of Vocabs, so that has been diverting me somewhat from keeping up with Grammar. So, because of that, it’s actually been quite a while since I’ve added new Grammar. But I continue to do reviews for existing grammar of course.