How many fan leafs needed

How many fan leafs would you need to keep on a harvested plant for it to beable to re veg?
The reason I ask, coming to the end of a grow and me and a friend would love to try replanting the 4 plants outdoors afterwards and check on them around the end of summer as kind of an experiment.

Thanks in advance.

Harvest the top 2/3 leave the rest bud and all. Some reveg fairly easy and others I have lost.