Android Pay has a 30-day limit on gift card adds it turns out. This means users aren't able to add more than a certain amount of gift cards to their Android Pay app every 30 days. As for the limit according to one user you can only add 10 gift cards to the app within that 30-day time frame and Google doesn't seem to have any explanations listed anywhere as to why this limit is in place.
In addition to the monthly limit on gift cards that can be added in there is also a 5 card total limit on merchants too. So if you're adding a total of 5 cards each for two different merchants in the same month, that's your entire limit for the month, meaning you would have to remove the cards from the application as you use them up to make room for new ones in the event that you needed to add more. Although the limitations being in place is a bit strange it might not necessarily affect a large number of users unless a sizable portion of them are carrying more than a few gift cards in the app each month.
Even with there being a limit on gift cards, loyalty cards are a different story as there isn't a limit for these on a monthly basis, so you can have loyalty cards for pretty much every single store you shop at regardless of how many that is and it won't be an issue. There also doesn't seem to be a limit on how many credit or debit cards can be added into the account, though most users probably aren't going to have more than a small handful of these sorts of cards to add anyway. Google is starting to branch out and expand the Android Pay compatibility by region as well as with more banks in the U.S. Google most recently added a list of new banks and credit unions to their Android Pay support list back in the middle of April, and they just recently updated the app to include support for PayPal accounts, meaning users will soon be able to draw from their PayPal balance when using Android Pay at merchants.