The concrete noun "command" as a synonym for an order, edict, or imperative given to someone is always countable:
He uses different commands to control his dog's behavior.
The general gave three different commands to his lieutenants before the battle began.
However, the abstract noun "command" can also be synonymous with authority, knowledge, or domain of control. When used in that sense, it is not countable, as in the examples you provided:
He has a good command of English.
The tower provides a wide command of the neighboring hills.
I highlight the distinction of concrete vs. abstract usage, because abstract nouns are often not countable as a general rule.