I tend to vary how many lessons depending on what is in the queue. For radicals, I typically try to get all of them in one set of lessons.
For kanji, I usually limit myself to 10 per day since I find that I struggle on reviews when I do more than that since I’m learning twice the information, meaning and reading, compared with radicals.
Once I get to vocab, I usually do 20 per day. I’ve found that a lot of the vocab tends to reinforce a reading I’ve already learned so I don’t have to remember as much and can handle a few more reviews.
I’ve found that this approach usually leads to a nice ebb and flow for each level where I tend to have more reviews at the beginning and end of each level when I’m reinforcing vocab and readings from a previous level or the end of the current level. But I get less reviews in the middle of the level when I’m usually learning the most new material and am the most likely to need a little more time breathing room to remember everything. I also feel like I rarely go through a long stretch without new lessons using this approach.