If it’s grammar you’ve studied previously and are comfortable with, you can be pretty aggressive with adding new lessons to your reviews— maybe 1-2 dozen per day. Just be aware that anytime you add a bunch of lessons at once (or knock out a bunch of reviews at once), they’ll all resurface again in similarly large batches.
When I’m going through completely new grammar, I usually go for 1-3 lessons per day unless they feel particularly easy, then maybe I’ll add more. I always add whatever looks easiest, which makes every single step forward feel non-intimidating.
The review system works best IMO if you can visit for small review sessions multiple times per day; check in frequently, but don’t spend very much time on each visit.
(Also, when lessons have similar English translations, or use the same Japanese vocabulary word, avoid adding those lessons to your reviews simultaneously. Otherwise, you might find yourself constantly confusing the similar grammar points for much longer than should be necessary.)
(Also also, I don’t know what the default setting is, but I’d recommend setting Ghost Reviews to “Minimal” to start with.)