First, we should note that there is a certain oddity in modern English in the nomenclature for numbers like “one thousand million”, “one million million” etc. The modern English pattern differs from most of Europe (as well as from its earlier usage in English):
|Number||Continental Europe||Modern English|
As you can see, the Continental European convention is to alternate between -ion and -iard, whereas the modern English convention uses just the -ion suffix.
Another important difference in comparison to other European languages is that the words “hundred”, “thousand”, “million”, etc. are never used in the plural when pronouncing a number, for example:
two million one hundred thousand (2,100,000)
two millions one hundred thousands (2,100,000)
We only use the plural (“hundreds, thousands, millions”) when referring to an unspecified number of objects of the given order of magnitude. Compare:
Thirty million locusts swarmed over the city.
As to whether to use “and” between “hundred” and the rest of the number, both “two hundred and fifty” and “two hundred fifty” are correct. The variant with “and” is more common in British English whereas the variant without “and” is more common in American English.
Also note that, when speaking about the number of objects or people, we usually say “a hundred/thousand/million”, rather than “one hundred/thousand/million”.