Locking cells in numbers
The short answer is, no, however you can lock a table.


To lock a Table, select it and then, from the Menu bar, click: Arrange > Lock


The longer answer is, if you want to protect* a cell or concurrent group of cells, do the following:


Overlay the target cell or concurrent group of cells with a rectangular shape.
Open the Inspector, click the Graphic tab, move the Opacity slider all the way to the left (0%).
Lock the shape in place, from the Menu bar, click: Arrange > Lock


This allows the contents of the cell or concurrent group of cells to be seen but not able to be modified unless the graphic shape is unlocked and either moved out of the way manually, set it to the back from the Arrange menu or remove it.

To relock the cell or concurrent group of cells bring the graphic shape back to the front from the Arrange menu or manually move it back, if it was moved, and then lock it again the Arrange menu.

Note: * While the method proposed herein keeps one from directly mouse clicking a cell under the translucent shape, it does not stop one from clicking into an adjacent cell that is not under the translucent shape and tabbing to a cell underneath, then modifying it. This method is meant as a workaround to keep one from using the mouse directly on a cell underneath and as a reminder the cell(s) in question is(are) meant not to be modified as easily as any wide open cell.