Hi
As soon as you created your ISE cluster, you just need to import licenses in primary device, that's it.
Asking how many licenses you need for tacacs means you don't have yet all licenses and in this case the new 2.4 licensing method applies, which is a license per tacacs node enabled.
I believe old skus are now end of sales and can't order them anymore.
Thanks
Francesco
PS: Please don't forget to rate and select as validated answer if this answered your question