Learn Finnish: Yksi mauste riittää.

In Finnish, "to be enough" or "to suffice" is just one verb: riittää. You don't need a separate word for "is" because the verb itself carries the whole meaning.

How would I make this sentence negative to say "One spice isn't enough"?

You would say Yksi mauste ei riitä. Notice that the verb riittää undergoes consonant gradation when it becomes negative, changing from a strong double -tt- to a weak single -t- in the form riitä.

How can I use the current lesson's grammar to say "Not even one spice is enough"?

You would attach the clitic -kään to the word yksi (one) to create yksikään (not even one). Because this requires a negative verb, the full sentence becomes Yksikään mauste ei riitä. It uses -kään instead of -kaan because the word yksi only contains front vowels.

Numbers usually force the following word into the partitive case. Why is mauste in the basic nominative form here?

While numbers from two onwards do require the partitive case (like kaksi maustetta), the number one (yksi) is an exception. It behaves like a regular adjective and simply matches the case of its noun, so both words remain in the basic nominative form here.