Each TNG eWallet account allows you to link up to 15 TNG cards. Please refer to this link on what type of cards are permissible to be added to TNG eWallet.
Below are the articles that may be useful for you :
How do I know the status of my Touch ‘n Go card?
Can I remove my Touch ‘n Go card from the eWallet?
I have removed my Touch ‘n Go card from my eWallet. Can I add it back?