I have 20 websites and I want to know what plan service I need to cover all 20 sites with this ad solution.
Is the $10 / mo base free per website or is the cost collective per impressions?
You can serve ads to any number of websites from a single AdPlugg account, there are no additional per-website costs. You would just have the $10/month base fee for the account and then pay for the total number of impressions across all sites. So the cost for a single AdPlugg account that serves ads to 20 websites that get 1M impressions each would be the same as the cost for having a single AdPlugg account that serves 20M impressions to a single website.
That being said, if the sites run totally different ads, it may be better (for organizational purposes) to create separate accounts.
If you decide to keep everything together in a single account, you can use AdPlugg's filters to filter within your AdPlugg dashboard based on site. You might also want to use different Zones on each site. You can then (optionally) create Placements that target Zones on two (or more) sites.
How you do it is all up to you, depending on what your goals are and how the sites are related. Feel free to contact us for recommendations based on your specific situation, etc.