One way to do it is to Assert that row n exists while  row n+1 does not exists with xpath, something like :
assertElementPresent | //table[@id='mytable']/tr[10] | |
assertElementNotPresent | //table[@id='mytable']/tr[11] | |
to verify table with id mytable does not have 10 elements

Another way is to use the SEL:storeassertXpath extension to count columns
assertXpath | count(//table[@id='mytable']/tr) | 10