It's not so much "at one time" -- rate limits aren't just about concurrent connections but about the number of connections over a certain time interval, like per minute and/or per hour.
Anyway, Marketo doesn't offer a built-in feature to throttle sends at this level of granularity. You can gateway some Marketo sends through a self-hosted server which has throttling applied, though this is a heavily technical task.
Given the increased focus on ABM activity, this is more important than ever to be able to configure "throttle" settings when sending to a single domain:
If you have a dedicated IP address you can file a Support ticket to see if our Deliverability team has any options to help. I believe they can assist in some whitelisting options.