word choice - How the AmE speakers call a building's apartment

What do the AmE speakers call the apartment number 13 in a building which has e.g. 20 apartments when they are mentioning to their postal address:

Apartment / unit 13 - No 10 - Harvard street - Massachusetts - United states

1) Apartment 13

2) Unit 13

Answer

First of all, we usually give an address like this:

10 Harvard Street
Apartment/Unit/Condo/Suite/Room 13
City Name, Massachusetts
zip code (if needed)

The street number and street name comes first. Then the unit number. However, many condos have their own unique street address.

You use that order whether giving someone your address or writing your address down on paper. If you are going to mail a letter, use the official two-letter abbreviation for the state (MA for Massachusetts).

As for whether it is okay to say condo, I don't know. First off, different parts of the USA refer to things differently. In some places an apartment is a single bedroom in a shared condo. In most places, an apartment is a rented home (consisting of a number of bedrooms, bathrooms, living room, dining room, kitchen, etc) in an apartment complex.

However, if you are talking about a specific unit (#13) in a building of 20 "apartments" (see, here I do not know whether you mean 20 rooms or 20 apartment homes), then say the appropriate word, then the number: Apartment #13, Unit #13, Condo #13. But in many condos they also just say Apartment Number and Unit Number.

In an office bulding, we can say Suite 13. We don't say Apartment 13 or Unit 13 for office building: these are used for places where people live. Unit can also refer to a storage unit, where one stores all the junk that they cannot fit in their house. :)