last time it was only 10 channels...
anyways, even though there are modules that make chanserv or other anope clients sit in channels, anope was not designed to support it and those modules basically join the clients without worrying about side effects like desyncs.
imo there s no point in keeping empty channels open either, cuz well.. they r empty and only empty chans showing up in /list doesn't look particularly good either... also keep in mind that mIRC will hide channels with less then 3 users in then from /list by default.
and if you have 300 chans and over 10.000 users, you wouldn't want a bot in every empty channel anyway since it would consume large amounts of resources that can be spend in a better way.