How can I get unique ID of asset publisher?

You can use the portlet instance id for referring each asset publisher on the page.

${themeDisplay.getPortletDisplay().getId()}

Now you will be able to get unique ID of asset publisher

<#assign portletID = themeDisplay.getPortletDisplay().getId() />
<#assign splitID = portletID?split("_") />
<#assign assetID = splitID?last />

${assetID}

Here ${assetID} is the Asset publisher ID.