The number of devices you can use to watch Sling TV simultaneously is based on the Sling TV subscription. If you have subscribed to the Sling Orange package, you can avail single-stream feature. Likewise, the Sling Blue plan allows you to watch service on three different devices at a time.
The same goes for Sling Orange + Sling Blue package that enables you to stream your desired content on 1-3 devices simultaneously.