Unless Instagram have posted this somewhere, it is impossible to know. Anyway, I don't think it is really relevant, what if travel is the most searched? Put a photo up with travel hashtag and it will disappear down the list in a matter of minutes, for no one to ever see again if the searched for travel (unless you can hit top 9). I think we can make an approximation that hashtag size roughly follow search volume, if we don't consider trending hashtag that is valid for a short period of time.