How many dragon heads are there in Minecraft?

I found a dragon head in the latest version.

Does this only exist once in all of Minecraft or is it very rare?

The dragon head can be found in end settlements. As far as I know, there isn't just one per world

There are several and with certain equipment they are also quite easy to find

I currently have 2 stacks of these things. But I've already sold some to sacrificial kiddies. So they are not rare.

Here is a page to change your skin: https://www.minecraftskins.com

Just log in and then choose a skin.