Not necessarily either. It depends on the language (how the phrases are built) of the commands. If you have many commands that sound similar to each other then it will be more difficult for the engine to tell them apart. However having tens of thousands of commands won't necessarily be a problem as long as they do not sound too similar to each other. The greater the number of words in a single command, the more reliable it will be and usually the higher the confidence will by. This is one reason that using a prefix can help.
Note that using a prefix essentially doubles the number of commands available, because you can say each command either with or without the prefix.
Having an extremely low number of commands will tend to increase false positives because the engine will generally assume that you are trying to give it one of the commands it knows and pick the closest one. If you only have 5 or 6 commands it will tend to fire them off frequently even if you don't say anything very similar to the actual command phrases.
The best rule is to make sure you don't use really short commands like "up". This will always be a problem.
