One syllable is the shortest a name can be and still be a name. Said at a distance it arrives whole, which is the entire argument: a two-syllable name loses its second half to wind and traffic, and a three-syllable name gets shortened by you within a fortnight anyway.
The one rule that overrides it
A single syllable is also what most commands are. Sit, stay, down, no — pick a name that rhymes with one of those and you have made both harder to teach. Check your shortlist against the cues you intend to use before you check it against anything else.
Counted from the spelling
Syllables here are computed, not estimated: the count comes from the vowel groups in the name, with silent endings handled, which is why it agrees with how you would say it out loud rather than with how it is spelled.
Every count on this site is a tally of real registrations: pet licences and shelter intakes published as open data by New York, Seattle, Austin, Montgomery County, Cambridge, Zurich, Basel and three Australian councils. Popularity here is the average share a name holds across those registers, so a name common in a small city is not buried by one common in a large one. Nine of the eleven sources are North American, so read the ordering as naming in those places rather than worldwide.