Right now I have 8 strains on hand - there are few more I want, so I think I'll end up keeping 11 total. I just keep them in 100 mL glass jars in the fridge, so they take up very little space. Every time I make a starter, I pour 100 mL of it into a jar, and it goes back in the fridge. So the yeast only see ~1.035 starter wort, which should keep mutation to a minimum. Not quite as good as banking the original culture in glycerol and only using each tube once, but a lot easier.