Reset invoice counter every month

Yes, that’s possible. The most reliable method for this is to keep the sequence for each month in separate database tables, you can use this code snippet:

add_filter( 'wpo_wcpdf_invoice_number_store_name', function( $number_store_name, $store_base_name, $date, $method, $document ) {
	$year  = $date->date_i18n( 'Y' );
	$month = $date->date_i18n( 'm' );
	return "{$store_base_name}_{$month}_{$year}";
}, 10, 5 );

There’s an alternative method here, but it’s more prone to errors because if for some reason you need to create an invoice for last month (or next month), this will trigger a reset each time you switch between months.

it works great!
Thank you