There are two situations in which MSP will "overload" a given ministry at a given service, scheduling more volunteers in that ministry than are explicitly required.
The first situation is if there is a team of volunteers in the ministry that is too large to fit in any service time. In this case, since the team would otherwise never be scheduled, MSP will overload services in order to schedule the team. This feature is often used to schedule teams of Ushers when each team is a different size and exactly one team is required per service. In this case the services should be setup to just require the number of Ushers in the SMALLEST team, and then the appropriate services will automatically be overloaded when it comes time to schedule the larger teams.
The second situation where services are overloaded is if the "Never split up family members in this ministry" option is turned on for a particular ministry, and one family has too many members in the ministry to schedule them all at any given service. When it is time to schedule the family in that ministry, the number of required volunteers at a service will be expanded so that the entire family may be scheduled. This feature is often used for Greeters when for each service exactly one FAMILY should be scheduled, not just a single volunteer. In this case each service should be setup to just require one volunteer in the Greeter ministry, and the "Never split up family members in this ministry" should be turned on for the Greeter ministry. Then services will automatically be overloaded and an entire family will be scheduled for each service in the Greeter ministry, as opposed to just a single volunteer. (Note that if you just have a few over sized families and you would NOT like services overloaded so that those families can be scheduled, but you want to keep all other families together in a given ministry, you can turn on the "Allow members of this family to be split up in ministries for which families are flagged to always be scheduled together" option in the Edit Family dialog just for the over sized families. That will ensure that services are not overloaded.)