Some cards are needed even if you don't track sales for individual customers
At least one customer card is required to enter invoices. If all your sales are to one-time or walk-up customers, you might create a "Cash Customer" card and use it for every sale you make.
Employee cards are not required, but they contain useful information and can be helpful for tracking if you assign custom lists or fields to them. Personal cards are for your reference only and are not required.