In general what is the max percentage can someone give to Maaser? one fifth is the max?
10% is Maaser.
20% or a fifth is Chomesh.
The common practice (there is a Machlokes whether it is Min Hatorah, Miderabanan or a custom which was accepted and obligates us due to general Minhag, ancestors Minhag or it is like a Neder) is to give Maaser – Ten percent – of all income that one makes to Tzedaka. However some go beyond that and give a Chomesh – one fifth – of their income to Tzedaka.
In general, the maximum amount one should give to Tzedaka is a Chomesh, one fifth of their income, however there are situations where this does not apply.