Numbers does not support most Excel array formulas (identifiable by the presence of 'curly brackets' ( {} ).
LARGE, in Number accepts only a single number for its second argument, so you'll need an iteration of the LARGE function for each of the values you want to include in the sum:

SUM(LARGE(C2:T2,1),LARGE(C2:T2,2),LARGE(C2:T2,3),LARGE(C2:T2,4),LARGE(C2:T2,5),LARGE(C2:T2,6),LARGE(C2:T2,7),LARGE(C2:T2,8))
You can also choose to build a list of the numbers to be included in a single column array in a separate column, or row on the same or a different table, Then SUM that one dinension array with a second formula on the original table.

The formula on the right is entered in A2 of the single column table on the right, Table 2, and is filled down to row 9.
The basic SUM formula on the left is in cell B3 of the original table.
Regards,
Barry