Wondering is there a limit for sending the activation emails (other than the API rate limit)? We have around 150k users that are being migrated, and will send all 150k users their account activation email on a specific time.
Jazz is right, you can send the activation emails to all new clients via your API and there is no limit, except the 20 requests per sec for shopify plus and 2 requests per sec for shopify standard stores.