Unfortunately, you won't find anything official for him - no certifications, and there's probably too many different compilations released over the years for any in particular to have accumulated significant sales on Soundscan or anything like that.
I did read that 'Tutti Frutti' (from 1955) sold 3 million worldwide, probably in Murrels' Book of Golden Discs. It would be nice if it was true, but it's probably wishful thinking.
'Long Tall Sally' (from 1956) could have been a million seller. It was a US R&B #1, and also charted strongly in the UK, reaching #2 or #3 depending on which chart. He was still popular in the UK after his star waned in the US, with 'Baby Face' hitting #2 in 1959.
He re-recorded his biggest hits in the Sixties. The first Little Richard album I bought turned out to be these re-recordings !
My God, it's full of Beatles albums !