Lesser should be sufficient. What's your preferring method to generate your wallets? Do you use seed words or separate private keys?

If you use separate private keys, 4 units is enough since a SteelDisk can hold up to 170 characters. If you use seed words, you can generate wallets for different coins with the same seed phrase; so even 1 unit might be enough to store your holdings.

Check here (https://bitcoin.stackexchange.com/questions/54956/is-it-possible-to-use-the-same-bip39-list-of-seed-words-to-create-addresses-for) and here (https://iancoleman.io/bip39/)