zpool replace healthy drives questions

I have a 8-disk (4TB each) raidz2 vdev (no other vdevs) and plan to upgrade all of them to 10TB each and expand the capacity at the end. The existing drives have no errors or SMART problems even though they are 5 years old.

I just acquired an 8-bay external enclosure (USB) so I can replace disks without unplugging the existing working drives.

Questions:

1. How many drives should I buy and replace at the same time? I haven't bought all replacement drives yet. I would like to spread out the buying to avoid bad batch.

2. Am I correct to assume that I still have at least two drives worth of redundancy during the replacement process?