If your org has a concurrency limit of 10, a maximum of 10 calls will be started at a single time. The rest of the calls will be queued and retried a few minutes later as your concurrency slots become available. To increase call rate, you need you increase your Vapi org concurrency limit.
Note that concurrency limits are on the Vapi side. Your telephony provider (e.g. Twilio) may have other rate limits.