When setting up DNS records for clients moving to hosted email, typically there are multiple MX records created.
I have a client using Office365, and the DNS section of the Admin tool only lists a single MX option.
Is this something I should be concerned about?
Nope, Office 365 utilizes a single MX DNS record. Behind the scenes Microsoft uses their network to spread the DNS and MX servers around the world. They perform geo-DNS resolutions for you to find current up servers and the closest one to you.
Nothing to be worried about.
From Microsoft