Otocinclus, How Many?

Hi guys, my tank is going well and I am now looking to add Otocinclus. My only issue is that some sites say they need to be shoaled, however no site says with how many.

 I got 6 pandas and love how they swim around together they look too cute (apart from that loner who seems to hate his own kind) off btw is it normal for pandas to clean other fish? some of my gold fish love chilling at the bottom while the pandas give em a scrub down....

Anyway, back to the otos how many should I get, 200 L tank 100cm long.

Thanks for the question and Sue's helpful reply - I'd been wondering the same thing (purely for future reference as it's not currently an option for me as I'm fully stocked).

Re the loner fish, you might well find that, over time, this all changes.  I've noticed many different configurations over the 9+ mths I've had my tank established: x-ray tetras having a loner one for a while, the two females paired off, two separate mini-shoals of 3; the pygmy cories being very much independent, then pairing off but not necessarily in the same pairs; the harlequins shoaling in a group of 5 with the small sixth one pairing off with the similar-sized cories or copying the scuffles between the tetras or copying the dart-to-the-surface behaviour of the cories even although there's no food there.  It can be "all change", and re-arrangement of aquarium décor can play a major part of this and as to who decides to occupy which part of the tank.  It never gets boring, that's for sure!