I have a garnishment order for monies owed to a financial instituion and payable to the Minister of Finance, sent to a local court clerk.

What is the maximum amount of garnishment ih this case? Is it 20%?

Its 50% of the gross wages.

I read in another question and answer session that it is only 20% unless the garnishment is for family support? The it can be as much as 50%?

Sorry, its 50% for Family support. But its 20% for debts
Garnishment of wages paid by the Federal Crown (BP-439E) (http://dsp-psd.pwgsc.gc.ca/Collection-R/LoPBdP/BP/bp439-e.htm)