A simple example of how to adjust the myCRED Lottery add-on to add an option to set a maximum lottery entries before the lottery is closed.

Max Lottery Entries

/**
 * Step 1 - Add new ticket preference
 * In this example we will insert this under "Participation" section
 * of the lottery edit screen.
 * @version 1.0
 */
add_action( 'mycred_edit_lotto_participation', 'add_max_lottery_entries_settings' );
function add_max_lottery_entries_settings( $lottery ) {
	$tickets = get_post_meta( $lottery->id, '_tickets', true ); ?>

<h2>Tickets</h2>
<table class="table table-bordered" style="margin-bottom: 0;">
	<tr>
		<th style="width: 25%;"><label for="mycred-lottery-tickets">Max</label></th>
		<td><input type="text" name="mycred_lottery_tickets" id="mycred-lottery-tickets" value="<?php echo $tickets; ?>" class="short" /><br /><span class="description">Max number of tickets available before this lottery closes. Leave empty to disable.</span></td>
	</tr>
</table>
<?php
}

/**
 * Step 2 - Save new ticket preference
 * @version 1.0
 */
add_action( 'save_post', 'save_max_lottery_entries_settings', 1 );
function save_max_lottery_entries_settings( $post_id ) {
	if ( ! current_user_can( 'edit_posts' ) || ! isset( $_POST['mycred_lottery_tickets'] ) ) return;
	$max = sanitize_text_field( $_POST['mycred_lottery_tickets'] );
	
	update_post_meta( $post_id, '_tickets', absint( $max ) );
}

/**
 * Step 3 - Hook into myCRED
 * Hook in and close the lottery when entires reach this
 * new max.
 * @version 1.0
 */
add_filter( 'mycred_lotto_after_play', 'limit_max_lottery_entries', 10, 2 );
function limit_max_lottery_entries( $reply, $lottery ) {
	// If the entry failed, leave now
	if ( $reply['status'] != 'done' ) return $reply;

	// Count lottery entries
	$count = $lottery->get_total_entries();

	// Limit
	$limit = get_post_meta( $lottery->id, '_tickets', true );

	// Do not use if not set
	if ( $limit == '' ) return $reply;

	// If we are over the limit, close the lottery preventing more entries.
	if ( $count >= $limit )
		mycred_lottery_do_close( $lottery->id );

	// Always return something or we brake everything
	return $reply;
}