Question

Or should the total amount of cells equal to columns * rows?

A table with different amount of cells on different rows seems to pass the W3 Validator.

Answer 1

That does not violate the definition. But you should use the colspan attribute to expand the column to the whole row:

<table>
  <tr>
    <td>Row with</td>
    <td>2 cols</td>
  </tr>
  <tr>
    <td colspan="2">Row with only one col</td>
  </tr>
</table>

That will also help if you have borders around the cells.

You can even have tables with a column that expands to two rows:

<table>
  <tr>
    <td rowspan="2">First column in row 1 and row 2</td>
    <td>Second column in row 1</td>
  </tr>
  <tr>
    <td>second column in row 2</td>
  </tr>
</table>