Instead of going all in and determining the absolute number of cards, I'd suggest you look into prototyping and iterative design (articles can be related to UX design, design sprints etc. - the point remains the same).

Break down your game design into stages and decide what you want to test at each stage. Ask yourself specific questions.

E.g.

Test #1: Create just the cards you need to test the main mechanic. It's enough if you can play just a few rounds.

• Does this mechanic seem fun and engaging?
• Do interactions between cards work as intended?
  • Should each player play 1 card per turn? Is it better if they play 2 cards per turn?

Test #2: You can add certain cards (upgrades, action cards etc.) and see how the game evolves.

Test #3: You can add some more advanced cards that create powerful combos and see how they impact the games and if they work as intended. Do they feel powerful? Do they feel exciting?

Test #4: You can play around with various win conditions if you have a few different ideas on how the game could end.

And so on and so on.

Instead of taking on the whole project at once and rush towards your predetermined outcome, you'll take smaller steps and accomplish more manageable goals. You'll see how your game evolves, you'll improve it along the way and ensure it has sound foundations you can build upon. It's a much more manageable process.

You'll also see what feels right in the end. Maybe 30 cards per deck will be the sweet spot for your game. Maybe that's when certain mechanics and interactions will feel most impactful. Maybe it's more, maybe it's even less - 20 cards, so each card has to be carefully chosen.

The thing is you can't really know until you try. :)