What does Macbeth mean when he says, "There's not a one of them but in his house/ I keep a servant fee'd" (3.4.132-33)?in Macbeth

What this is telling us is that Macbeth does not really
trust anyone.  He is telling his wife that he has paid spies in the homes of every lord
in Scotland, presumably.

This comes up because Macbeth says
that Macduff will not come when Macbeth orders him to come.  He has not actually ordered
Macduff to come and so Lady Macbeth wonders why he knows that Macduff will not obey. 
That is the point when Macbeth speaks the line you
mention.

When he says "not a one of them" he is referring
to the lords.  By saying "a servant fee'd" he is referring to someone he has paid (given
a fee) to work for him as a spy.