WP Query filtering by custom category not showing all relevant posts

@jacob-peattie answered the question in his comment – I used the wrong key to set the number of posts my query generates. Swapping numberposts with posts_per_page solved the issue.

Here’s the final query:

@php 
    $cats = get_sub_field('reviews_category'); // gets the custom field categories
    $args = array(
                'post_type' => 'testimonials',
                'posts_per_page' => -1, // this line updated
                'order' => 'DESC',
                'orderby' => 'meta_value',
                'meta_key' => 'date',
                'tax_query' => array(
                    array(
                        'taxonomy' => 'testimonial_categories',
                        'field' => 'term_id',
                        'terms' => $cats
                    ),
                )
            );
    $query = new WP_Query($args);
@endphp