Seb, if I had a 10 row grid question (assuming row direction), you could create a constructed list for the rows that will do the following ...
1/ Randomise the list
2/ Trim the list to your liking
Assume the grid is Q1 with a row list called "Q1RowList" which consists of 10 statements / items.
The constructed list would look something like this ...
You will now see only 5 randomised statements / items displayed at Q1.