Zapper is a great tool for getting an overview of assets across multiple wallets. As an example, a16z holds UNI tokens across 11 different addresses. In the portfolio below, all these wallets are bundled into one portfolio on Zapper which shows that a16z owns 38m UNI tokens.
There might of course be additional wallets not tracked but nonetheless this is a great way to get a broader overview.
NOTE: In contrast to Debank, Zapper does not support all Defi protocols. If wallets have tokens deposited into smaller defi protocols they will not be displayed on Zapper. So if you want to be 100% sure, use Debank for the individual wallets.