$ man starting-contacts

Start with at Least 5,000 Contacts

For a single campaign launch, load at least 5,000 contacts into the sending platform. This is the per-campaign minimum that ensures enough volume to warm infrastructure, test messaging, and generate early signal.

5,000 contacts per campaign spread across 18 sending accounts at 25/day means the campaign runs for roughly 11 days. That's enough to cycle through the full list, measure open/reply rates with statistical significance, and decide whether to scale. Fewer than 5,000 and the data is too thin to optimize. You're guessing, not testing.

When I set up a new campaign in Instantly, I load a minimum of 5,000 qualified, enriched, MX-routed contacts. I don't start a campaign with 500 contacts "just to test." That tells me nothing. I start with 5,000, measure for 2 weeks, and make data-driven decisions about copy, targeting, and channel mix based on real metrics at meaningful volume.