Learn Finnish: Leivotko sinä paljon?

This happens because of a rule called consonant gradation. When conjugating a Verb Type 1 like leipoa, we take off the final -a to get the stem leipo-. When we add the ending -t (meaning you), it closes the syllable. This forces the strong letter p to soften into a v, making the word leivot.

Why is there a -ko attached to the end of the verb?

The -ko is the Finnish question particle. To make a yes/no question, you put the verb at the very beginning of the sentence and attach -ko (or -kö) to it. So, leivot (you bake) becomes leivotko (do you bake?).

If leivotko already shows who is baking, do I need to include the word sinä?

No, you do not have to! The -t ending on the verb already tells us you are the one doing the action. Finnish speakers often drop the words minä (I) and sinä (you). Including it here just adds a little emphasis, but Leivotko paljon? is also perfectly correct.

Could I ask this by putting paljon at the beginning of the sentence instead?