I think 4 is a good number for a full one gallon pot, but 8 is probably fine too. Depends on the plant and the root system and the patience level and how soon you want to see results. It will definitely set them back, but they will usually catch up by year 3.

I think a lot depends on the plant. And how well the plant handles having its roots disturbed.

I had a shasta daisy alaska that I divided this spring. It was probably 12-14" diameter. I must have divided that into close to 25 divisions. And at least half of those divisions bloomed this year - granted each division only had 1 or 2 blossoms. But considering I bought this as a little 3" plant 3 years ago - I would say I got my moneys worth!

Siberica and Day Lillies are other plants that seem not to mind being divided into tiny divisions. Last year I was dividing tons of siberica. After dividing, I ended up with tons of spare root clumps. I buried the root clumps, and lo and behold got plants this year! Whenever I divide day lillies, if I have any additional little bulb things, I just plant them somewhere. It may take 2 or 3 years to get blossoms from these little bulbets - but I'll take delayed blossoms over the compost heap.

Carex 'Ice Dance' - divided my original plant this year into 9 divisions. I divided it last year and took just two small divisions to plant in another bed I was creating but the plant seemed rejuvenated after that and grew to 1 1/2 times the size it was before I divided it the first time. I decided to divide it again based on the way it increased after the first time. Gave some to my daughter and planted the rest as an edger to a bed on the north side of my house. They're all looking healthy, including the small bit of the parent plant which is once again roughly the size it was when I first bought it. From what I've observed, chances are good it'll be back to full size again next year. Since I probably paid around $4.99 for the original plant, a dozen divisions seems like a pretty good deal for that price.

That's funny that you mention Carex 'Ice Dance'. I once bought a very pricey small pot of it. $9 for a quart. When I went to plant it, I was shocked at the little pieces of plants with barely any roots. It must have been recently divided and was really not ready to be sold, but that didn't stop them! So I planted it anyway. That was March 2003. Since then it has grown to 2' wide and been divided and pieces given away a gazillion times!

Yes, definitely depends on the plant.

oops... did I say March? I meant May!

For a mature hosta, I divide them into one good chunk for me for replanting and then make 30 one-eye divisions from the rest of it for sale.

To get them that small, it is best to remove all the soil and tease apart the roots.