When using a yeast cake from another brew, is it possible to use too much? If so, what is the limit for a 5 gallon keg?
There is not really any hard and fast rule, I would say it depends on a number of factors.
- Age of yeast cake
- Gravity of last brew
- how rapidly you want to get this one going
In a professional setting you would check cell count, viability and a number of other factors. But unless you have stain and a microscope this is not really an option.
The older the yeast cake the more I would use as the viability and vitality of your yeast are likely to be lower.
Same for the ABV of the last brew the higher the abv the less healthy your yeast will be.
If from an old cake with high ABV maybe use 3 cups from a very fresh brew at ~4% then as Frazbro suggests a single cup of cake should more than suffice.