What is the meaning of "The number of units in the LSTM cell"?
Most LSTM/RNN diagrams just show the hidden cells but never the units of those cells. Hence, the confusion. 
Each hidden layer has hidden cells, as many as the number of time steps.
And further, each hidden cell is made up of multiple hidden units, like in the diagram below. Therefore, the dimensionality of a hidden layer matrix in RNN is (number of time steps, number of hidden units).