Knapsack with an even number of items.
You can keep two tables with $n$ rows and $C$ columns: 

$DP_{even}$ that saves the best knapsack solution with an even number of itens. 
$DP_{odd}$ that saves the best knapsack solution with an odd number of itens.

To fill $DP_{even}$ you look at the previous best solution of $DP_{odd}$ plus a new item, or of $DP_{even}$ if you don't take the item:
  $$DP_{even}[i, j] = max(DP_{odd}[i - 1,j - c[i]] +  v[i], DP_{even}[i - 1, j]) $$
The same idea goes for $DP_{odd}$
$$DP_{odd}[i, j] = \max(DP_{even}[i - 1,j - c[i]] +  v[i], DP_{odd}[i - 1, j]) $$

Your solution will be in $DP_{even}[n, C]$