Simple Q about the Domain Mapping Plugin

Hi Guys,
I am looking at setting up multisite, and I will be offering a paid service, setting up websites for local small business, and I obviously want them to be able to use their own doman name.
Can I restrict the number of domain names they can use using the domain mapping plugin? or once they have the capability of adding in one domain they can then add multiple domain names. Matbe I have the wrong impression on how the plugin works but it seems like the user has control over adding in their domain names.
I have looked around but havn't found any info on this scenario.
Thanks
Jeremy