Add a max custom interval: https://gist.github.com/bernattorras/26b2a7e22ad0b772e36f718431a6a781

add_extra_period_intervals.php

<?php

add_filter('woocommerce_subscription_period_interval_strings', 'add_extra_period_intervals', 10, 1);
function add_extra_period_intervals($intevals){
	
	$max_intervals = 20; // The maximum number of intervals you want to display
	$intervals     = array( 1 => _x( 'every', 'period interval (eg "$10 _every_ 2 weeks")', 'woocommerce-subscriptions' ) );
	foreach ( range( 2, $max_intervals ) as $i ) {
		$intervals[ $i ] = sprintf( _x( 'every %s', 'period interval with ordinal number (e.g. "every 2nd"', 'woocommerce-subscriptions' ), WC_Subscriptions::append_numeral_suffix( $i ) );
	}
	return $intervals;
}