where does it say i can ONLY have one transport. its not listed in the bible.
Check the entry for dedicated transports in a unit that can have them: Like regular marines on page 134. "May select A rhino or A razorback." It is singular. Only one transport, (of any type) for a single unit no matter the units size.
and the SM codex states that the unit may choose razorback or rhino, if the unit numbers 10 models, it may choose drop pod. so sure, its a rhino or razorback, but it doesnt say, like it did in the old codex's, " if squad numbers 6 or less, it may choose razorback. if squad numbers 7 to 10 it may take rhino". so its still possible to take a razorback for the squad. Nowhere does it say that if you have more than the max allowed can you NOT buy the transport in the new codex.
Yes, you can take a razorback for a unit of 10 marines, they just cannot embark into it until they number 6 or less models.
Hope that helps.