While digging through the code for T89217: Should be possible to rename or merge change tags, I found that entries to the tag log have the following parameters

$logParams = [
	'4::revid' => $rev_id,
	'5::logid' => $log_id,
	'6:list:tagsAdded' => $tagsAdded,
	'7:number:tagsAddedCount' => count( $tagsAdded ),
	'8:list:tagsRemoved' => $tagsRemoved,
	'9:number:tagsRemovedCount' => count( $tagsRemoved ),
	'initialTags' => $initialTags,
];

As far as I could see, the 7:number:tagsAddedCount and 9:number:tagsRemovedCount parameters are never used or read directly, but only passed to the messages to allow for {{PLURAL}} logic. I suggest that, rather than logging this information, it be calculated when needed using the same count method.