How many disk can be added in Raid 10

As many as the number of ports you have to plug them in to.

It actually depends on the card or controller. You would have to look at the company's site or instructions to find out.

Any multiple of two that is greater than two, up to the number of available ports. RAID 10 is a mirror (two times) of a RAID 0 (two or more drives). The bare minimum is four, two RAID0 of two each.

Technically, you could mirror more than two times. How about 9 drives in three RAID0 of three drives, with three copies? It would be a little silly, but you could do it.

I've run RAID 10's of 20+ drives, it all depends on your controller.