Standard card pack sizes

That's what I would do.

Card decks come in units of 55 cards because you can fit 55 cards in one big printed sheet of card stock.

You don't need to make the decks in your game to be of 55 cards each, but you might want to set yourself two goals:

a) use as few cards as possible, to reduce potential production costs, thus making the game more attractive to the publisher
b) make the total number of cards in the game a multiple of 55, or a bit below a multiple of 55 (no problem with a discarding a few unused cards, but too many will seem like a waste, and you'll probably get a better reception if you manage to reduce the total size to one less deck of 55).

Your game currently has 4 decks of 68 plus one of 60. That amounts to 332. 55x6 = 330. Removing two cards from your game would mean it needs (from a production standpoint) six standard decks of 55. Hence, it might be good to reduce the total count to 330.

That said, it's all up to how the decks are produced. A standard deck of poker or bridge cards requires 54 cards, so it makes sense to use a sheet of card stock with room for 55 (5 rows of 11 cards), but a company producing games might be more flexible. Using slightly smaller cards will make room for 56 cards (8 rows of 7). On certain stocks, this might not be possible, due to fiber orientation issues that would result in the cards being too prone to be bent by riffle or dovetail shuffling. As you can see, there's not just one absolute answer.

Decks of 55 is the standard, but you shouldn't worry too much about that. If removing two cards is easily workable, I say you should do it. If it ruins the game balance, I would say don't. At least not until you have a publisher interested in the game asking you to remove those two cards. Publishers are not so shortsighted that they will just check the card count and say "too many cards, reject the game". They will let you know if the game is worth publishing, but needs to use less cards. They might ask you to use a total of just 275 or even 220 cards if that's what they need to reach a certain production cost required to be competitive. So it makes little sense to constrain yourself with a "magic" multiple of 55, if that means sacrificing game quality.

It will depend on the printer and the size of the cards. 56 per sheet is common.

But there are some tricks that printers can do to make different numbers work. For example they can print an oversized deck on three sheets and split it so that each deck would have 84 cards. This is cheaper then wasting half a sheet. As I am not a printer I can’t get into details about it, my information is second hand.

A designer should be able to work within the practical restraints and adjust the game as needed. Games that don’t fit in to the guidelines may be considered UNPUBLISHABLE. This simply means that it is too expensive to publish and they can’t expect to make money doing it.

You don’t necessarily need to stick with 56 cards per deck as long as the total number of cards in the entire game is divisible by the number of cards the printer has per sheet.

If your total card count is (4x68)+60 = 332 then 6 sheets of 56 cards will be needed to produce a set. 56x6 = 336 leaving you 4 extra cards. As long as the printer has the proper sorting equipment its should be fine.

What you want to avoid is wasting cards. If your game had 4 cards left over and the publisher was planning to produce 5,000 copies that wastes 20,000 cards, The equivalent of 357+ sheets. Often these need to be removed from the machine manually by the operator, which adds expense. That is why you sometimes find blank or advertising cards included with games.