Each phone number you have in your account can make roughly 100 calls per day. Take into account the amount of contacts you have in your campaign and how many you'd like to be able to call within a day. The amount of agents you have dialing on a campaign also has an effect in regards to the amount of phone numbers you should have.
Example:
You have a list of 100 contacts and they each have 3 phone numbers.
That would be 300 phone calls you'd need to dial, which would mean you need at least 3 phone numbers for that campaign.