What is the difference between the Professional and Agency plan?

The only difference between these 2 plans are the number of site activations for each. These 2 plans have the same list of features but a different number of sites you can activate your license on.

With a Professional plan, you can have up to 5 site activations. With an Agency plan you can have up to 1,000 site activations.

See: What features are included in Professional and Agency plan of WPI?