Counting number of posts with Category B in Category A

Make use of the WP_Query class, namely the tax_query and fields parameters. Get the count from the $found_posts property. Please note, this is exemplary code.

$query = new WP_Query( [
  'post_type' => 'games',
  'tax_query' = [
    'relation' => 'AND',
    [
      'taxonomy' => 'game_status',
      'field' => 'slug',
      'terms' => [ 'beaten' ],
    ],
    [
      'taxonomy' => 'console',
      'field' => 'slug',
      'terms' => [ 'switch' ],
    ]
  ],
  'fields' => 'ids',
] );

$count = $query->found_posts;