Multiple Patronuses

Question

In Harry Potter, we do not see or hear about anyone conjuring more than one Patronus (that I remember), except for Professor McGonagall:

'We must alert the other Heads of House. You’d better put that Cloak back on.’

She marched towards the door, and as she did so she raised her wand. From the tip burst three silver cats with spectacle markings around their eyes. The Patronuses ran sleekly ahead, filling the spiral staircase with silvery lights, as Professor McGonagall, Harry and Luna hurried back down.

-Deathly Hallows - page 480 - Bloomsbury - chapter 30, The Sacking of Severus Snape

How is it that she was able to produce more than one?

We know that it takes a seeming amount of energy to produce one: https://scifi.stackexchange.com/a/8528/21267

Also her Patronuses (Patronii?) were small; could this have helped?

Answer 1

Patronuses were also used for sending messages - especially between order members. See Messenger Spells

In your quote - McGonagall states that she wants to alert the other heads of houses - i.e 3 different individuals - about the events that transpired. That was how she was able to produce more than one since the intended recipients were more than one.

Regarding the amount of energy required - I don't believe that a happy memory is required to produce a messenger spell - since no incantation is required. The patronus is only an embodiment of the message - to identify the sender.

Edit: Found the right archived link https://web.archive.org/web/20110806214330/http://www.jkrowling.com/textonly/en/faq_view.cfm?id=99