if you compare „last touch instances“ vs. „visits“ without any segment, it is possible that visits is higher than instances. this is due to the fact that the last touch keeps its value for 30 days (depending on configuration). that means if a user returns without touching a channel, it keeps the channel from the last visit. therefore it‘s counting a new visit without increasing the instance metric!
if you only want to see visits where the user touched a channel you need to use a segment (eg a „visit“-segment where “last touch instances“ equals 1). with this segment applied the metric for instances should be at least the same as visits, in most cases it‘s higher (since a user can touch more than one channel during a visit)