No every D is 1 harq , so D1D2D3SU1 , D4D5D6SU2 (ack D1D2D3).
In U2, you will get ack for D1D2D3.
Therefore these are free and eNB will reuse 1,2,3.
Heavy DL will be like DDDDDDDD, you need to check what are supported format.
Even with bad RF, you cannot precisely control failures to trigger 16 harq process - it will be link failure before that.
Precisely you reach 5 ( 0 to 5) because of above explaination