You want to find the total Number of cards you need, and then figure out how many unique cards you have. Now figure out how many of each unique card you will need. The more cards the higher the chance that card comes up.
Think of it like this: A deck of playing cards has 52 Cards. 4 of those cards are Aces. Whats the odds that you deal an Ace in a single hand? Im not going to get into the odds here, but this is the type of thing you want to think about when building your decks of cards. How often do you want a particular card showing up? Is it a common card or a rare card? If a card is to powerful, maybe the answer is simply making the card more rare rather than changing the card itself.
In my game, I have several decks of cards. I have a combat deck that handles how combat is resolved. I have a Cargo Deck that is a randomizer for salvaged valuables after combat. And I have an Event Deck that creates random events like storms, pirates, etc...
I will need to determine how many of each card I want in each deck before I can build my Data Sheets other wise you end up doing work twice....just like I did the other day. =)
Here is a look at the first run of cards: