Can HTML table have variable number of cells on rows?

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

Yes, it can. In table > tr > td, td is contain the content. Td reference here: http://www.htmlcodetutorial.com/tables/_TD.html. In that reference, you can see that TD has 2 attributes that is: colspan and rowspan. By using those 2 attributes, a table can have different amount of cells on different rows.
Demo:

     

          
a
          
b
     
     
 
          
c
     

And

     

          
a
          
b
     
     
 
          
c