You'll find that the sound modification in Japanese is a fairly regular and predictable process, once you've got used to the language.
That's one way to put things, the other is to think of this change of pronounciation as
liaisons , the same way French or other languages have it.
You will then have :
-hitotsu (or ichi) hiki becoming i
ppiki because tsu or chi in front of hi doubles the consonant (cf.
ha hi hu he ho /pa pi .../ba bi ...)
- ni hiki
- san biki because the n causes the h to become b (like in the phonetic rule m,p,b , n having the role of m here, like in shimbun)
- yon hiki is an exception to the rule (not yon biki), though I think
yoppiki could be possible (but never/seldom used), shihiki is rare
- gohiki
- roku hiki or roppiki (roppiki being more correct, like roppon /roku hon)
ku acts like
tsu
- nana hiki (shichi hiki is rare, shippiki logical but not used)
- hachi hiki or happiki (better)
- kyu hiki
- ju hiki or juppiki (better) or even
jippiki
Looks (or sounds) insurmountable but go back to what I like my TV said, it's true ...