Wouldn't 'Berapa nomormu?' mean 'How much is your number?'
This is one of those things that can't be translated word per word because they're idiomatic expressions. You just have to memorize how these questions are expressed in Indonesian.
How much is your number?=berapa banyak nomormu?
In english, in many contexts this question would often be asking specifically for a phone number. Is this also true for indonesian?
Apakah nomormu is not good?
No, it has to be "Berapa".
Wouldn't this be what are your numbers? (since kalian is plural you?)
Why does "apa nomormu?" is not correct?