there are excpetions to how counters are pronounced depending on what number they are used with
It depends on the fact that some numbers end with an unstressed syllable formed by a stop consonant and an "i" or "u". In these cases, when the following counter begins with a voiceless stop or an "s" (note that in Old and Middle Japanese "h" was pronounced "p" [ɸ], so it counts as a stop, and sometimes it has become, intervocalically,
w, like in the particle は), the vowel was dropped and the two sounds merged. It happens also with
jū (it was pronounced
zipu in Middle Japanese). The process is very regular.
h (p in Middle Japanese):
一 => iti + pai > itpai > ippai
六 => roku + pai > rokpai > roppai
八 => pati + pai > patpai > happai
十 => zipu + pai > zippai > jippai
百 => hyaku + pai > hyakpai > hyappai
with a preceding
n it becomes
b:
三 => san + pai > sambai
千 => sen + pai > sembai
万 => man + pai > mambai
common counters with h are
hai, hon, hiki, hako 杯,匹,本,箱.
w assimilates with 六, 八, 十. A common counter is
wa 話 (for example
roppa).
w becomes
b after 三 千 万. A common counter is
wa 話 (for example
samba).
Other letters don't change after
n.
k assimilates after 一, 六, 八, 十. Common counters are
ko, ken, kai 個,軒,階 (for example
ikko).
s assimilates after 一, 六, 八, 十. Common counters are
satsu, shū 冊,週 (for example
issatsu).