Learn Spanish (Spain): ¿Tienes cuarenta o cincuenta libros?

In Spanish, the numbers for the "tens" from 40 to 90 all end in -enta. So, while 4 is cuatro, 40 becomes cuarenta. And while 5 is cinco, 50 becomes cincuenta. Recognizing this ending makes memorizing the tens much easier!

Would the numbers change if I was asking about feminine objects, like magazines (revistas)?

No. Unlike the number one (uno/una), the tens from 30 to 90 do not change to match whether the noun is masculine or feminine. You say cuarenta libros for masculine words and cuarenta revistas for feminine words.

Why is there no word for "Do" at the start of this question?

Spanish does not use extra helper words like "do" or "does" to form questions. You simply take the verb, which in this case is tienes (you have), and put question marks around the sentence to ask "Do you have?".

Why is the word for "you" (tú) missing from this sentence?

In Spanish, the ending of the verb usually tells you who is doing the action. The verb tienes specifically means "you have" (informal). Because the verb already gives this information, adding the word tú is optional and usually left out unless you want to emphasize who you are talking to.