Can I limit Kindle purchases to only 1 device on my account?
I am assuming that you are talking about Kindle Fire which runs (heavily modified) Android OS.

All purchases made on a Kindle are linked to whatever Amazon account you are signed in with, not the actual device.  If you wish for your purchases to be separated, you will need to have a different Amazon account for your second Kindle Fire.

Now the problem is that there is no way to share purchases between accounts. As far as I know, you cannot add a second Amazon account to the Kindle (like you can with multiple Google accounts on plain Android.)