"How many are they?"
Translation:वे कितने हैं?
Kitne goes before the noun that you want to know the number of. Eg: 'kitne log' (how many people), 'kitne seb' (how many apples), 'kitne rupay' (how many rupees) etc.
'वे' is not used in this way similar to how you can't say 'how many they' in English.
To say 'how many of them', you'd say 'उन में से कितने'.