Once past a handful of accounts, memory stops being a system.
WHAT TO RECORD PER CLIENT
Client name and contact details cPanel username and primary domain Package and any custom limits Where the domain is registered, and its expiry date Renewal date and price
Which services they bought: hosting only, hosting plus maintenance, design
Backup arrangement agreed Notes on anything unusual about the setup
WHY IT MATTERS
You will need to find an account by domain when a client emails "my site is down" without mentioning which site. You need renewal dates to invoice on time. You need domain expiry dates, because a lapsed domain takes a client offline even though hosting is fine. If you sell the business, or someone else takes over support, the records are the business.
WHERE TO KEEP IT
A spreadsheet is adequate to start. A billing system such as WHMCS holds it for you once you outgrow that.
KEEP IT OFF THE SERVER
A record of your clients stored only on the hosting account is lost in the same event that loses the hosting.
