When we say "development device" we mean the PC/Mac where you install the SDK.
If you have 1 Windows PC and 1 Mac, both will have Marmalade SDK running on it, you need 2 licenses.
If you have only 1 Mac for everything, you need only 1 license.
The license can be release/reassign to the other machine, so if your dev machine somehow went down(hope that won't happen), you don't need to buy another new license. But in cases you might not have a chance to release it while the dev machine is still alive, please contact us for help.