Generally, numbers are treated like adjectives.  They usually go after the word they modify.  (Again, most orcs would probably ignore this rule.)  You will also notice that the Ring Verse places them before the word they modify (ash nazg . . . ).  This may have been done for poetic reasons.

Ash, one

Krul, two

Gakh, three

Zag, four

Kraak, five

Rut, six

Udu, seven

Ski, eight

Krith, nine

Gal, ten

*Note that 21 would be krulgalash, 24 = krulgalzag, and so on.
 

Exercise:

Translate the following numbers into Black Speech:

34 __________________________   58 ____________________________

62 __________________________   71 ____________________________

22 __________________________   87 ____________________________

103 _________________________  245 ____________________________

9068 ________________________ 4862 ____________________________