How can I display the number of post associated to a category in Twig/Timber WordPress? [closed]

You’re assuming that get_terms() returns an object with a count property, but it doesn’t.

From the docs:

Return

(WP_Term[]|int[]|string[]|string|WP_Error) Array of terms, a count thereof as a numeric string, or WP_Error if any of the taxonomies do not exist. See the function description for more information.

Also, the way you’re calling get_terms() is deprecated. There should be only one argument, an array that includes the taxonomy name.

So here’s how I’d update the code:

$orderby = 'name';
$order="asc";
$hide_empty = false;

$cat_args = array(
    'taxonomy'   => 'product_cat',
    'orderby'    => $orderby,
    'order'      => $order,
    'hide_empty' => $hide_empty,
    'exclude'    => 18,
);
$context['categories'] = get_terms( $cat_args );
// Gets the count in a separate call.
$cat_args['fields'] = 'count';
$total_in_term = get_terms( $cat_args );
$context['TotalInTerm'] = $total_in_term;

References

• get_terms()
• fields parameter