Long story short: It depends.For your Campaigns or Quests, you should first think of how many people you expect to be engaging with your game. Consider the target audience size, genre, popularity of the brand, and any potential marketing pushes you might make for the Campaign.
As a rule of thumb, it is good to start off with 100 keys. From there, you can see over time how many keys are being used up and add more accordingly. Keep in mind that depending on the platform, you can experience wait times for new keys. Don't wait for there to be 0 keys before requesting more from your distributor!
You should also take into consideration keysets. On Lurkit, you can add extra keys that will be given to creators, such as DLC or extra keys if you want Creators to play with friends for more multiplayer games. We've had a Campaign on Lurkit that gave out 10 extra keys to anyone who was accepted into the Campaign so they could give it to their friends and play together!
If you want to give out extra keys, then keep in mind that your total keys might run out faster and you'll need to scale that up with the audience.
For example, 100 keys with +2 keys per person means you'll need a total of 300 keys for the Campaign, and 3 additional keys for every user that is accepted into the Campaign.
To summarize, start with 100, then consider your goals, target audience, if you will give away additional keys, and keep track of the keys that are remaining.
12 keys remaining out of 14 for this Campaign in the image above.