Hi Mohamed,
If it is a Rack structure and movement of stock is in pallet then it is advisable to go for L-next empty bin.
For maximum utilization you can plan to go for mixed storage or addition to existing stock however in my opinion there are some
Cons of mixed storage / addition to existing stock
Since it is a high rack (assuming that vertically it goes upto one or two level) if you want to mix stock on the same pallet it will be a time consuming process and increase your overall putaway time
also during picking if you have to pick 2 unit from a rack which is on 4th high then bring the pallet down take out 2 unit and then put back the pallet on the same position or another (as per config) which is going to increase your overall picking time