There is no limit to the number of Contacts you can have on a single list in Constant Contact. There are some limitations on the API for how many Contacts can be added in certain periods of time and through different methods.
Bulk API allows for up to 20,000 contacts in a single request. These are batch jobs that are processed on our server after the request comes in. They typically take a few minutes to process, with larger jobs taking longer.
The Contacts API, for doing single Contact add/updates, is limited to 200 create/update requests per second. Since this is probably not the way you'd want to manage 100,000 Contacts, this limitation should not be a problem for you. I would highly recommend the Bulk Activity API for your needs.
Dave Berard Senior Product Manager, Constant Contact
View API documentation, code samples, get your API key.