set post limit at load more ajax wordpress

Something v basic might look like this

var ppp = 9;
var loaded = ppp;
var maxPosts = 50;
postData.append( 'action', 'loadmore' );
postData.append( 'paged', section_posts );
postData.append( 'posts_per_page', ppp );

if ( loaded < maxPosts ) { // check if we're below the max post number 
    const xhr = new XMLHttpRequest();
    xhr.open( 'POST', ajaxurl );
    xhr.addEventListener( 'readystatechange', function ( data ) {
        if ( this.readyState === 4 && this.status === 200 ) {
            loaded = loaded + ppp; // Add our per page to our loaded tally
            section_posts++;
            document.querySelector( '.box' ).innerHTML +=
                data.target.responseText;
        } else {
        }
    } );
    xhr.send( postData );
} else {
    alert( 'You have loaded the maximum number of posts' );
}

It’s not exact – it would still load ppp more posts if the loaded amount was 1 under for example. If you want to do something more you can put a secondary condition to check if you’re below the limit but would be above with the added ppp