How many billion or how many billions?
Both are grammatical, but the latter is far more common and idiomatic.
You usually use hundred, thousand, million, billion, or trillion with a final 's' after the determiner "many" if there's no noun after these numbers. For examples:
How many billions did he lose?
But if there's a noun after these numbers, you use them without final 's'. For example:
How many billion dollars did he lose?
Further, you use these numbers without 's' after a, one,two, three, a few several, etc. For examples:
A/one hundred dollars, six thousand people, several billion dollars, etc.
Also, ou use these numbers with a final 's' if there's no other number before them but there's the preposition "of" after them. For examples:
Hundreds of dollars, millions of people,etc.