Each set of Bitcoin private keys is capable of generating billions of public keys. These public keys are then transformed (through a mathematical process called hashing) to produce public addresses that you see on the Bitcoin blockchain and are visible in your wallet. Every single one of those addresses can receive Bitcoin. Each set of private keys you own is capable of producing its own unique, massive set of public addresses that you, and you alone, own. Anyone can send Bitcoin to those public addresses but only the holder of the private keys can spend Bitcoin from those addresses.
Many people like to compere private keys, public addresses, and Bitcoin wallets to email addresses. The private keys are your password, the public address is your email address, and the wallet is the email client you use (Gmail, Outlook, Yahoo, etc.). This analogy is useful except with Bitcoin each password (private key) that you own gives you access to millions of potential email addresses (public addresses) from which you can send and receive Bitcoin.
Don’t be concerned if your wallet consistently generates new Bitcoin addresses. That’s actually one of its features! Wallets generating and using new public addresses help protect your privacy from people snooping on the public Bitcoin blockchain. Remember that so long as you still hold the private keys to your wallet, you alone still own the present and future Bitcoin sent to any public address that your wallet generated. Neither wallet addresses nor balances can expire. Just make sure to keep your Bitcoin private keys safe, secure, and private.