Incorrect Use of wpdb::prepare()

You should try to replace the meta_value = $list->ID part with the placeholder meta_value = %d. Then use the following:

$subscriber_count = $wpdb->get_var( $wpdb->prepare( $q, $list->ID ) );

where the input argument $list->ID will be treated as an integer (signed).

From the Codex:

The query parameter for prepare accepts sprintf()-like placeholders.
The %s (string), %d (integer) and %f (float) formats are supported.
(The %s and %d placeholders have been available since the function was
added to core in Version 2.3, %f has only been available since Version
3.3.) Any other % characters may cause parsing errors unless they are escaped. All % characters inside SQL string literals, including LIKE
wildcards, must be double-% escaped as %%. All of %d, %f, and %s are
to be left unquoted in the query string. Note that the %d placeholder
only accepts integers, so you can’t pass numbers that have comma
values via %d. If you need comma values, use %f as float instead.

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