On levels 19 and onward you get approximately 130-170 lessons per level, so for the sake of simplicity lets say an average of 150 per level.
If you want to do 8 days per level, which is comfortably fast and doesn’t require doing reviews at 4 in the morning, the minimum amount of apprentice items per day is around 75.
You can do 75 items in the first 4 days (which must include all the radicals) and the other 75 items (including the newly unlocked kanji after guruing radicals) in the other 4 days.
This is all hypothetical for the sake of calculating the minimum amount of apprentice items, and requires 100% accuracy on the radicals and on the second batch of items.
In reality, there is no need to do 75 items per 4 days since your accuracy will always be below 100% so you’ll want to do as much as you can in the first 4 days and leave only the critical items for the other 4 days.
I think I consistently have around 150 apprentice which includes some leftover items from previous levels.
I average 8 days 9 hours per level on 97-98% accuracy.
People chant “less than 100 apprentice items” mantra but I think 150-200 is easily manageable if you have the time and energy to go faster.
Screencaps for proof
https://imgur.com/a/gvczA