How to find ISSN number of IEEE Sponsored (or IEEE related) Events?
You can look at this list https://www.ieee.org/documents/2005_present_list_of_titles.xls from IEEE of upcoming events, and events from 2005 to present with all corresponding publication numbers.

As a comment points out, many of the listings do not have ISSN numbers. However, you can look at line 10935 of the excel sheet and see your conference. 

IWCMC.  The IEEE conference number is 34432 and ISBN 978-1-4799-5345-5