Of course! Visitor Analytics is perfectly suitable for agency use.
The stacking offer was tailored to fit the use case of agencies that want to upsell website analytics to their customers. By stacking 4 deals, you'll be getting lifetime access to 15 websites, with a limit of 30k visits/month each.
In case your agency is big and you need more websites, you can create additionally accounts and stack 4 deals per account.
For each created website, you can invite your clients as collaborators - you can have an unlimited number of collaborators. The clients receive a registration link in their email and they can easily use that to sign up for an account and get access to their analytics.
You'll be able to set daily, weekly or monthly email reports that the clients will receive together with a link that leads them to their analytics dashboard.
And don't worry, the clients won't have access to any other site data other than the one they've been assigned to!