Starting from the initial position, the minimum moves you need to develop your game completely are:-
* 2 Pawn moves to let the Bishops out
* 4 Minor piece moves
* 1 Queen move
* 2 Rook moves, developed to a center file
* 1 King move, usually by castling, more often by O-O than by O-O-O
That makes a total of 10 developing moves. This holds for both sides, whether you're playing White or Black. Looking at it another way, you start the game with eight pieces. On their original squares the pieces are undeveloped. You need to make at least one move with each piece plus two Pawn moves for the Bishops. That makes ten moves.
The count isn't perfect, because it ignores some common situations. For example, developing the Queen often requires another Pawn move, and castling is not always so straightforward. Sometimes castling develops both the King and Rook immediately. More often a second move is required to develop the Rook (after O-O) or to safety the King (after O-O-O). Despite its inaccuracies, the count is useful to compare the deployment of your army relative to your opponent's.
I've found that counting the development moves is also useful in chess960. There are, however, several different considerations. I'll cover some of these in future posts.