Find minimum number of figures needed , so that no additional figure can be added?
You want to compute the independent domination number of a graph with one node per tromino and an edge for each pair of trominoes that overlap. You can solve the problem via integer linear programming, as shown in my answer here. Optimal values for $n \times 2n$ with $n\in\{1,\dots,10\}$ are $0, 2, 3, 6, 9, \color{red}{12}, 17, 22, 28, 35$.