I've noticed this in a few of your xpath, so I just want to clarify the @value meaning.
That @value means "attribute called value", and @name means "attribute called name". In your spawnings, you are using @name='city' correctly, as name="city" is valid in the XML node. However, you go on to use spawn[@name=maxcount]. There's no attribute call name in the spawn line.
The conditions in the [ ] are there to help you isolate which line you want to change, and can be any attribute on the line that is somewhat unique.
The following lines would work, and both will adjust the ZombiesAll line.
<set xpath="/spawning/biome[@name='city']/spawn[@entitygroup='ZombiesAll']/@maxcount">6</set>
<set xpath="/spawning/biome[@name='city']/spawn[@time='Day']/@maxcount">6</set>