My cable provider, (starts with an X), offers 9 HBO and 9 Showtime channels.
Exactly one of each is offered in HD, (really?)
If I purchase Showtime from Roku, how many Showtime channels will I get, and are they all in HD?
You should be asking HBO/Showtime themselves.
I deal directly with the Channels I subscribe to. You are not obligated to go through Roku.