What is the minimum/maximum size for an Objective Deck? Is it ten objectives, no more no less?

Minimum of 10 objectives, no maximum.

Ok. That is what I thought. What is the optimal number? I've been using 12.

Most people use exactly 10 (makes it slightly more likely to find important cards and objectives). 11 or 12 doesn't change the statistics drastically and may be the correct choice for some decks.

12 sets means 60 cards. You often don't get through half that in a game. However, if you want to double up on some unique units, it's understandable to add another set or two.