How to count dom elemnts with Cypress?

You are using p-multiselect-item as a class (leading dot) but actually it's a tag.

Try

cy.get('p-multiselect-item .p-highlight')  // find <p-multiselect-item> 
                                           // with children having class p-highlight
  .its('length')
  .should('eq', 2)

Have you tried this :

cy.get('@groupSelect').children('option').length

if not you have some answer here : How to count DOM elements in Cypress assertion