Two, with at least 30 days between them. The first sends the primary hook. The second, sent if there's no reply, can be shorter and more direct: "Sending one more note before I wrap up on my end. Happy to chat if the timing is ever right."
After two attempts with no reply, stop. A customer who has fully churned and isn't responding has either found a solution they're happy with or isn't ready to re-engage. Continuing to email them risks a spam complaint or a permanent negative impression. Keep the door open in the last message, then move on.
When a churned customer replies but doesn't commit ("not right now" or "we're revisiting in Q2"), reply once to confirm the timing and go quiet until then. "Makes sense. I'll check back in [Month]" is the whole reply. A single light acknowledgment keeps the door open without extending the sequence into territory where you become a recurring reminder.