50 slots may be too many... compromise at 30? hmmm
Let's say there's only one original route and another in a different perspective, i'm guessing unlocked after 1st clear, but basically I'm seeing only two complete playthroughs. That is assuming bonus content is attainable through normal playthrough.
So let's say for 10 hours of gameplay, there is an average of about 15-30(may be less) story based decisions, assuming there are only four possible endings(good, bad, normal and true) but may be restricted to only two (good & bad) or the boring single ending, and taking into consideration the process of determining the ending [i.e. number of positive decision vs negative decisions where as majority of positive decisions = good ending, majority of negative decisions = bad ending, an indecisive average = normal ending, and a series of correct story-wise decisions = true ending].
Here is an equation that I have devised.
n = 3/4(AX)+(AY)
Where:
n = the number of save slots necessary, rounded to the nearest ideal number
A = average number of choices per decision
X = average number of decisions in one playthrough
Y = number of possible endings
3/4 = playability constant placed
So if there were let's say 15 decisions to be made each having 3 possible answers, and there are 3 possible endings, the number of slots would be 42.75 rounded to the nearest ideal number would be between 40, 42, 45 and 50.
Just my random ramblings though. I made it up.