How can I specify I want output of which units in LSTM?
If you use:

LSTM(10 , return_sequences = True)


you will only get as output something of size (batch_size, num_timesteps, 10).  The "10" for LSTM unit-size is a hyper-parameter you should be tuning.  However, what you want is something the size of (batch_size, num_timesteps, num_features).  This is why you want to put a Dense layer after the LSTM layer.  That being said, you will also want to add a TimeDistributed wrapper around your Dense layer, so that a Dense layer is applied to every time-step (1):

TimeDistributed(Dense(num_features))


Now the output of the TD dense layer will be of size: (batch_size, num_timesteps, num_features), which I'm assuming is what you want.  You will have to set up your Y data to be of the appropriate size for this to work.  It will have to be of size (batch_size, num_timesteps, num_features).  You mentioned that you are curious to know how to use just part of it.  At prediction time, post training, you would call your model.predict and it will return something the size of (batch_size, num_timesteps, num_features).  You can just use numpy array slicing to access what it is that you want.