The ExtendSim License Agreement for an Individual license states:

"A single identified individual (the “Authorized User”) may use the Software. The Software may be used by the Authorized User only for the Company’s own internal business purposes or for your own personal use (if you are an individual), as applicable. Shared use, use by more than one individual, or use over a local-area network (LAN), a wide-area network (WAN), or the internet is prohibited under this License Option."

This means you are permitted one activation of your ExtendSim license for your own use. You may move your license from one of your computers to another at any time. But will have to deactivate ExtendSim on the first computer before it can be activated on a second as ExtendSim will not permit an additional activation. See Moving ExtendSim from one computer to another.

Details on activating and deactivating ExtendSim licenses can also be found at Installing & Activating an ExtendSim 10 Individual License.