Well, unlimited, theoretically, but it's one of those things where you have to decide how much of a hobby within a hobby you want out of it. Your yeast may a) mutate b) be contaminated c) need additional zinc (around gen 6-12). You can somewhat take care of "b" with an acid wash, though some wild yeast may be resistant. You can take care of "c" by -- surprise -- adding *the correct amount of* zinc to your wort; you need really really little of it, but you need it. You can probably take care of "a" by re-growing the colony from a single unmutated cell, but I've never done that, because that's where I drew the line with the hobby-within-hobby business.
I'd say forget yeast calculators because they don't know how many cells per mL your slurry has and are effectively calculating guess in => guess out. If using your process your lager completes fermentation in 7-10 days you pitched enough, otherwise you underpitched and you need to pitch more next time. The exception is if you're on the nth generation and have *not* added zinc, your fermentation may be sluggish or stall due to a lack of zinc instead of cells. If you warm-pitch, the pitch cell count isn't quite as critical as with cold-pitching.
