Numbers merge sheets

Do you mean to move one table from one sheet to another? Select the table and make sure no single cell in it is selected (click on the green circle on its top-left.). Then Copy, change sheet and Paste. If there are many objects (tables, charts, texts, shapes, etc.) you can also "drag a net" around all of them before the operation.

You can then delete the original sheet with the menu in the tab.

If you mean merge tables then you can copy-paste --

1. Add a new blank row to one of the tables
2. Go to the other table, select all its cells, and type command-c.
3. Click once in the first cell of the blank row you have just created in the destination table and type command-v or choose Edit > Paste and Match Style from the menu.

SG