Theoretically, there's no maximum number of contacts you can have in your Mailchimp audience. However, Mailchimp is a browser-based application, so there are a few things you should consider when you have a large audience.
Importing a very large audience into Mailchimp can take a while. If you're seeing this take an extremely long time, reach out to a developer to look into syncing your database with Mailchimp through the API.
If you import an audience of more than a few hundred thousand contacts into Mailchimp, keep in mind that some audience pages may load slowly.
If you try to perform a search across an audience of hundreds of thousands of contacts, it could be slow or unavailable depending on your audience size.
Mailchimp can theoretically send millions of emails from our servers per hour, but no internet service provider (ISP) would actually accept that much email so quickly. ISPs throttle emails to prevent massive spam attacks from bringing down their services. To work toward better delivery rates, we in turn throttle our delivery, based on each ISP. This results in slightly longer delivery times for very large audiences, but overall higher delivery rates.
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