There's no hard contact cap built into the software itself — it's self-hosted, so it's really bounded by your own server's resources (mainly database size and RAM for very large imports). Your license tier affects sending volume and feature access, not contact storage limits. That said, if you're importing more than a few hundred thousand contacts at once, it's worth doing it in a few batches rather than one giant CSV, mostly so you can catch and fix any row-level errors without redoing the whole import.