Use cien when 100 stands alone or comes directly before a noun.
Use ciento when 100 is part of a longer number:
So in this sentence, 100 is standing by itself, so cien is correct.
The accent mark matters because más and mas are different words.
So in this sentence, the accent is necessary because the word means more.
In most of Latin America, cien sounds roughly like see-en, but said as one syllable: syen.
A simple approximation for English speakers:
So the whole sentence sounds roughly like:
syen ess mahs keh dyes
Note:
Because the numbers are being used directly as numbers, not as nouns with an article.
Spanish does not normally say:
Instead, it simply says:
You may see articles with numbers in other contexts, but not usually in simple statements like this one.
By themselves, numbers do not usually change for gender in this kind of sentence.
So:
stay the same.
However, some numbers do change in certain contexts, especially uno:
And ciento can appear in forms like:
But in Cien es más que diez, there is no gender change to worry about.
This is a very common question.
Use más que for comparisons:
Use más de mainly with numbers, quantities, or expressions like more than three, more than a year, etc.:
So:
Yes. In this sentence, the numbers are functioning like noun ideas: 100 and 10 as concepts.
English does the same thing:
Spanish allows numbers to be the subject of a sentence in the same way:
So even though cien is a number, it can still behave like the subject of a sentence.