Question:

Is it possible to record the number of Other Attendees on a Call instead of recording each one of their names using the Attendee_List_vod__c field? Can it adjust the expense split in Concur based on that field?

Answer:

It is possible to map a number field to send the number to Concur but it ignores it in the splitting calculation. 

It is not possible to map a number field that automatically splits the expense to those additional unnamed attendees in Concur. Expenses can only be split between Attendees and Other Attendees.

Related Documentation:

CRM Help Documentation: Configuring Call Report Expense