How many numbers or digits can phone numbers have in them?

Answer 1

United States

Telephone service in the United States has now gone to a 10-digit phone number system. The parts of a telephone number in the US are the 3-digit area code, the 3-digit exchange then the 4-digit personal number. The area code and exchange route the call to the correct country, state, city, and possibly to an area of that city if the city has more than one area code.

A "1" is added to indicate "long distance".

Overseas

I don't know much about overseas phone numbers except that each has a country code. The US also has a country code if dialing from outside the US.
It varies from country to country. The country code can be one digit (+1 = US, Canada, etc.; +7 = Russia & Kazakhstan), two digits (+20 = Egypt; +27 = South Africa; +30 = Greece; etc.), or three digits (most countries). The national number, including the area code where applicable, can be anywhere from 4 to about 12 digits, but the total of the country code and national number cannot be more than 15 digits. In some countries, the number of digits is fixed; for example, all numbers in the US and Canada are a total of 11 digits: country code +1 (1 digit) + area code (3 digits) + local number (7 digits). In other countries, the number of digits may vary within the country, or even within a particular city or town.