When building Numbers spreadsheets it is important to remember that a row in a table is the equivalent to a record in a database. To build a billing system, you can put clients in one table and invoice items such as billable hours in another table. Then you can build a sheet with tables that use functions to populate an invoice. The requires some advanced functions like LOOKUP, INDEX, IF, AND, COUNTIF and more. But once you are done, you can easily add more records and create invoices.
Download the sample numbers file.