How to count posts with specific arguments

The easiest way to optimize this query would be to add 'fields' => 'ids' to the arguments. This way only ids of posts will be retrieved from DB (and usually that’s a big change).

$args = [
    'numberposts' => -1, 
    'post_type' => 'jobs', 
    's' => 'developer', 
    'tax_query' => ['taxonomy' => 'IT', 'field' => 'slug'],
    'fields' => 'ids'
];
$posts = get_posts($args);
$count = count($posts);

On the other hand you can use WP_Query instead of get_posts. This way you can set posts_per_page to 1, so only one ID will be retrieved and use found_posts field of WP_Query to get the total number of posts matching your criteria.

$args = [
    'posts_per_page' => 1,
    'post_type' => 'jobs', 
    's' => 'developer', 
    'tax_query' => ['taxonomy' => 'IT', 'field' => 'slug'],
    'fields' => 'ids'
];
$query = new WP_Query($args);
$count = $query->found_posts;