Check the section 6.1 in the Recommendation ITU-T E.164:
SERIES E: OVERALL NETWORK OPERATION, TELEPHONE SERVICE, SERVICE OPERATION AND HUMAN FACTORS
International operation – Numbering plan of the international telephone service
It says:
6.1 International ITU-T E.164-number length
ITU-T recommends that the maximum number of digits for the international geographic, global services, Network and groups of countries applications should be 15 (excluding the international prefix). Administrations are invited to do their utmost to limit the digits to be dialled to the degree possible consistent with the service needs.
It means that you have to count on 15 digits plus international prefix that in most countries is 00.