Learn Finnish: Hyvä suoritus vaatii monta lähdettä. - A good performance requires many sources.

The word moni (many) acts a lot like a number. When used as an object, it takes its partitive singular form monta. Just like numbers greater than one, monta forces the noun that follows it into the partitive singular. That is why we use monta lähdettä rather than a plural form like monet lähteet.

What is the dictionary form of lähdettä, and why does it have a double 't' at the end?

The dictionary form is lähde (source). Finnish nouns ending in -e form their partitive case by adding -tta or -ttä. So, lähde + ttä becomes lähdettä. Note that the 'd' remains a 'd' in the partitive, but if you used a case like the inessive, consonant gradation would change it to a strong 't' (lähteessä).

Suoritus translates to "performance". Is there a verb related to this word?

Yes, it is derived from the verb suorittaa (to perform, to carry out). In Finnish, adding the suffix -us or -ys to a verb stem is a very common way to create a noun that represents the action or the result of that verb.

Since this lesson covers comparatives, how would I change this sentence to say it requires "more sources"?