getAgentWorkload() from Omni toolkit can be used to get Agent configured and assigned Omni-Channel Capacity.

Sample Code:

<aura:component implements="flexipage:availableForAllPageTypes" access="global" >
    <lightning:omniToolkitAPI aura:id="omniToolkit" />
    <lightning:card>
        <lightning:button label="Get workload" onclick="{! c.getAgentWorkload }" /> 
    </lightning:card>
</aura:component>

JavaScript:

( {
    getAgentWorkload: function( component, event, helper ) {
        
        let omniAPI = component.find("omniToolkit");
        omniAPI.getAgentWorkload().then( function( result ) {
            
            console.log( 'Configured capacity is: ' + result.configuredCapacity );
            console.log( 'Assigned workload is: ' + result.currentWorkload );
            
        } ).catch(function( error ) {
            console.log( JSON.stringify( error ) );
        } );
        
    }
    
} )

Output: