50 USA States or fifty U.S.A. states

[Lifesnadir]

I wrote, "in 50 U.S.A. States plus Guam and the District of Columbia".  Is this correct when referring to States?  Or, should I spell out these words to "fifty U.S.A. states"? 

Also, should I use USA or U.S.A.?

Thank you,

Lifes

[Diana]

Any numerical reference exceeding ten should be written out. If you start a sentence with a number less than ten, that number is to be written out fully as well.

One, two...ten,11, 12...

Twelve cupcakes make up a dozen.

U.S.A is used when composing written text. 
USA is used to label items made in the United States or on letters/labels circulating throughout the United States.