1. loss of intervocalic (and initial) /s/ -> *kwosios became *kwoios
   
2. the labiovelar *kw normally gives /p/ in Greek, but in in some dialects it can result /k/ e.g κότε instead for πότε, ίκκος instead of ίππος.

Based on the above κοῖος should not be compared to standard Greek αριθμός but to ποσότης 'quantity'.

Another etymological suggestion is that it is related to the verb κοέω ‘to observe, learn, hear, estimate, take to account’.

Beekes (2014) considers the possibility of the word being an Anatolian or Pre-Greek loan.

See Hoffmann (1906), Kalleris (1954) and Filos (2015).