You cannot use more than 10 funds at a time in the backtest model. The backtest model only has sufficient room in columns B through K of the Calculations sheet for 10 funds.
You CAN backtest fewer than 10 funds. You will need to make sure the weekly closing price data for each fund starts in column B and fills in each column from the left without skipping any columns. Also, make sure the remaining columns are cleared of any old price data by highlighting the old prices and using Edit > Clear (NOT Edit > Delete!). Finally, you will need to put in some fake price data for the non-existing funds. Inserting “1” in all the unused price columns works. If you leave the unused columns blank or use zeros, you will get #DIV/0! errors.