Turn on Schedule, then Enable Campaign Manager. Two fields appear:

• Campaign Limit: how many messages go out per chunk. With 100,000 numbers, entering 10,000 gives ten chunks.
• Time Interval: the gap between chunks, selected from a dropdown.

This is worth doing on anything large, especially if the same route also carries OTP or transactional traffic. A single unthrottled promotional batch is the classic reason OTP latency spikes for everyone else on that route.