How to center text across multiple cells

My advice would be not to start from the idea that you want to bring Excel documents over to Numbers and keep working the way you have in Excel. Numbers shares many things in common with Excel, among them many formulas, but it has a different design philosophy. To see examples of effective document design in Numbers be sure to have a look at the built-in templates at File > New in your menu. As you will see it is common in Numbers to have multiple tables on one sheet, something that is not common in Excel because each worksheet is one big grid of cells.

In general I would avoid merged cells in Numbers, though as you've discovered the app does have that feature. You can unmerge them and turn off Wrap text in cells.

The text will then spill over into the neighboring cell if the neighboring cell is left blank.

Note that the name of the table at the top is naturally centered. That may be all that you need since you will often have several tables.

SG