Five configurations of the same mechanism: pressure-sensitive coatings that transfer a mark to every sheet below without carbon paper. The part count is decided by how many parties need an independent record at the moment the transaction happens. Filter by how many copies you need.

Original + 2 copies. Customer, field and office each need one.

Original + 1 copy. You and the customer, nobody else.

Original + 3 copies, the job passes through four hands.

Your parts, size and numbering. The presets do not match your process.

Original + 4 copies. Five parties hold the same record.
Count the parties, not the information. Two parts covers a transaction where only you and the customer need a record. Three is the most ordered because it matches how most work actually flows. Customer, the person doing the job, and the office that invoices from it. Add a part only when a fourth party genuinely needs an independent copy at the moment the form is filled in.
Be realistic about the physics above three. Four parts transfer cleanly with a ballpoint and firm pressure, but gel and felt-tip pens spread the load and the bottom sheet suffers. Five is marginal by hand and only really works with a hard-tipped pen on a firm surface. If the form is machine printed rather than handwritten, that limit does not apply at all. Sequential numbering is worth adding to anything that will later be queried or reconciled.