How many LSTM cells should I use?
The minimum number of training examples is what you have up there:

$$4(nm + n^2)$$

For more information refer to this article:
Refer to this link if you needed some visual help:
Number of parameters in an LSTM model

The number of units in each layer of the stack can vary. For example in translate.py from Tensorflow it can be configured to 1024, 512 or virtually any number. The best range can be found via cross validation. But I have seen both 1000 and 500 number of units in each layer of the stack. I personally have tested with smaller numbers as well.