Okta does not currently have a setting for concurrent sessions. Depending on what you are trying to do, however, there may be a different setting or configuration that might work for your needs. Can I ask about your use case?
We don't support any concurrent session controls (as far as I know) today. However, I'd recommend reaching out to your Okta contacts to talk through your requirements to see if there's another option for you that is within Okta's existing products.