The limit for most single-channel sms gateways is around 30 messages/minute. This is as far as I know a limitation of the SMS technology. Some vendors have multi-channel sms gateways, where you can add multiple sim cards to get a higher throughput.

As Bert said, DataMiner will queue the messages that cannot be sent immediately, so bursts of messages can be handled without problem, if you have a constant high volume of messages you will need to look at a hardware solution.