Probably the Colegio de Abogados membership number--in the U.S., at least, "Bar number" is sufficient.

Attorney Bar Number
www.mdd.uscourts.gov/attorney/attybar.asp

Attorney Name or Bar Number. Search Tips: Do not use nicknames. Use either a first initial or proper first name. Sample Search Phrases: ...
Advanced Search - Armen Janian - Scott Sirlin - Paul L. Brimberry
members.calbar.ca.gov/search/member.aspx - Cached - Similar -
The State Bar of California
Check an attorney's bar membership record ... My State Bar Profile Member Number Password First-time user? Forget your password? ...
Attorney Search - Member Services - Bar Exam - MCLE
www.calbar.ca.gov/ - Cached - Similar -
Attorney Inquiry by Bar Number
Attorney Inquiry by Bar Number. Bar Number: Please reset form before a new inquiry. Inquiry by Name. Please enter a valid Bar Number.
www.co.bexar.tx.us/liveweb/dc/dcatn02.asp

Your user name will still be your bar number.
pro.wsba.org/

This is probably what is meant. Número de inscripción/registro del colegio de abogados.