Great question. There is technically no limit on the number of contacts you can upload into Constant Contact. It does impact the pay tier you're in so you'll want to be mindful of that when importing. If you want to see what pay tier you're in and what the next tiers look like and cost you can find it under "Plans & Pricing" after clicking on the human silhouette.
If you want additional help understanding how the number of contacts you import will affect your pay structure I recommend reaching out to our Billing department which you can find on our Contact Us page. They are available from 7AM to 9PM EST Monday through Friday.