Automacally buy seats when SSO users sign up

SSO helps to centralise account management in a single place, and other services we use simply pick up new users automatically and adjust the pricing correspondingly. Is something like this available on GitHub? We are on the Enterprise plan. Currently, I either need to pre-buy a bunch of seats in advance (which is wasteful since they are not being used), or do it manually every time a new joiner tries to log into our organisation for the first time.