How to get only ONE category of “Portfolio” posts to display on main page rather than ALL categories?

Question

Wyck is correct. You can find the answer you seek in the wonderful WordPress documentation.

But, ’cause why not, here’s the answer for you:

You’re very close, you just need to change your “tax_query” array to include the term you’re requesting. Like this:

    'tax_query' => array(
                    array(
                      'taxonomy' => 'portcat',
                      'field' => 'slug',
                      'terms' => 'events'
                    )
                  )