Eleven

It's an affirmation. The riddle isn’t describing something, it’s simply telling you how many letters are in each word.
What contains four letters. Sometimes contains nine letters.
Never contains five letters.

Bed

9

6 ÷ 2 = 3, 1 + 2 = 3
3 (3) = 9

10

1+1+1+1 = 4
1+2+1+2 = 6
1+3+1+3 = 8
1+4+1+4 = 10

55

10

Time

A towel

A stamp

Seven thousand five hundred is the correct way to say 7500.

Seven and five ARE thirteen is the correct way. Except that seven and five is actually 12