There are exceptions, but IMO 8+ (ideally more) is a great way to keep many loach species, because they form quite complex hierachies.
I have 7 Dwarf Chain Loaches, adding 2 more to the original 5 that I got for a steal (<£30 from a great fish store) changed the behaviour of the group and made them far more outgoing. I'm looking to get a few more once my fish chaos is not so crazy! There is a definite pecking order in the group and the "shadowing" (two fish mimicing each other if a zippy dance for a few seconds) is brilliant to watch.
I have one male Indian Redtail Squirrel Loach, the only one of six that made it through the heatwave of 2010 after "Vickiandkev" from here got them for me from Wholesale Tropicals. I'd love to get a group again with the experience and tanks I've acquired since 2010, they can be having a right go at each other one day and share a burrow they dig between them the next.
I used to have 8 Weather Loaches, they quickly grew from skinny ~6cm large "Kuhli" size to ~15cm and the thickness of my middle finger. You could throw a couple of hand spans round the group all burrowed in the sand during the day, with their heads popping out, at night they would do laps of the 5x2x2 midwater (often in pairs/trios).
