Seeing as how there are 150 species each numbering from hundreds of millions to billions in population, I alway like to think that humans are in the minority and the ships we've seen are the exceptions.
Like today not every human out there will want/have to enlist into service, most will want their own lives and careers, etc away from Starfleet. So I like to think that humans make up at the very most 10% of active service personnel, but that's just the way I like things in my head.
Commander Austin Harris, First Officer, Deep Space Nine (by FltCpt. Bossco)
8.01 - Darkest Before Dawn
(Chapter 8 added, 12/09/2015)