If you rinse and collect your yeast you can get many generations out of it. And a yeast cake has enough yeast for another 2-3 batches of beer. And those 2-3 jars of rinsed yeast can all be used, collected and rinsed again to make 2-3 new batches of yeast. I generally will use a yeast for about 5 generations at the most. I’ve read that after 5 generations mutations can begin and at that point you want to dump the yeast. But collecting, rinsing and reusing yeast is an extreme cost saver.
This is how I do it.
http://billybrew.com/yeast-washing