Here is how Keras documentation defines an epoch:
Epoch: an arbitrary cutoff, generally defined as “one pass over the entire dataset”, used to separate training into distinct phases, which is useful for logging and periodic evaluation.
So, in other words, a number of epochs means how many times you go through your training set.
The model is updated each time a batch is processed, which means that it can be updated multiple times during one epoch. If batch_size is set equal to the length of x, then the model will be updated once per epoch.