Some of these numbers, especially for Italy and Anzac, are way over what is really used in an average game.
I would recommend that you start playing with the pieces that you already have and use plastic tokens or any other marker for the units that you don’t have.
That way you will see what pieces you really need and those that don’t get that much use.
If you wait until you have a “complete” set before you start playing, you will never get the feel for this and it will cost you a lot of money to buy pieces that may just stay in the box.
However, take into account that most battle pieces are sold in sets of five by HBG.