I have used some lager strains over 20 generations without an issue and had some that improve for a few generations and then stress out/mutate/go off kilter. I always use yeast nutrient. If you can be super sanitary, there is no reason that most yeasts can't go 5-6 generations without incident. I have overpitched on occasion, resulting in some lightly smoky and clovey phenols, so I try to use no more than a third of a fresh slurry for ales and half of a fresh slurry for lagers. I use a bit more if the yeast has sat a couple weeks after harvest and make a starter if more than a month has past from harvest. Writing the strain and harvest date on removable painter's tape stuck to the vessel makes tracking things easier. Keep the lids a little loose so residual CO2, if any, can escape and store in the fridge under beer. Some guys pour off the beer and add a little fresh wort from time to time to maintain viability...but I don't. I just throw old yeast away after a few months. Lastly, I like to grow yeast up by batch size, rather than making starters all the time with new yeast. Start with a smaller batch of say a gallon or two and progressively step up in size to get to the full batch size (10 gallons for me, typically).
Good luck with your yeasting!