Not directly. JIRA Service Desk is not an independent application, it's a layer that sits over the top of a project or projects in JIRA and makes them behave like a helpdesk.
It will work fine for one of your JIRA systems. It's not going to be so good for the other 20, because they don't have their own JSDs with portals.
Two practical solutions in a broad sense are:
1. Merge your 21 JIRA systems
2. Set up synchronisations (code or add-ons needed) so that your Service Desk can work one one JIRA and the issues flow into the others.
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