Is there a limit to how many canvas emails someone can send per second, minute, or hour? If there is, what is that limit?
Can you clarify what you mean by "Canvas emails"? People can't really send email directly through Canvas. Most communications in Canvas are either through the Canvas Inbox, which is Canvas "internal messaging" tool, or through Notifications, which the recipient may choose to have delivered via email, or via the Canvas app, at the interval they choose (immediately, daily, weekly...)
For more information about the Inbox, see https://community.canvaslms.com/t5/Canvas-Basics-Guide/What-is-the-Inbox/ta-p/55
For more about how Notifications work, see https://community.canvaslms.com/t5/Student-Guide/How-do-I-manage-my-Canvas-notification-settings-as-...