I'm so confused. Why is NYUMBA "house" singular and NYUFA "cracks" plural, yet both of them begin with NY- which I thought was a plural marker? I guess specifically what I'm asking is, is NYUMBA from some other noun class and that's how it's able to be a singular word starting with NY? What is the plural of this word? Before starting this section, I thought "house" was NYUMBANI. What's the difference between NYUMBA and NYUMBANI? Thank you for any help on this!
Holger's answer is correct wrt. the -ni suffix. It's a locative marker, and shows that the marked noun is the location where something is happening.
Talking about the plural prefix, the situation is not as simple as it seems. Most nouns have a plural prefix, and sometimes a singular prefix. But there is a noun class that has neither a plural nor a singular prefix. The noun class that is often called the i-/zi- class, or also the n-class, has no prefixes to the noun itself. You can only see whether it's is plural or singular by looking at context, or by looking at verb agreement. The word "nyumba" belongs to this class. The word "nyufa" belongs to the u-/zi- class, or also called u-class. This is a strange class, with words that sometimes have no plural, and acts as mass nouns (e.g. upendo - love), and as with ufa, it gets the plural form from the n-class. So in their singular form, the two words can be said to be in different classes, but in the plural form, they belong to the same class (or at least, they have the same form).
This was a longer post than I anticipated, but the short version is: nyumba belongs to the n-class of nouns, while nyufa is the plural form of the u-class of nouns.