how many songs will the IWatch 1 hold

Hi

The music playlist size on Apple Watch is initially limited to 1 GB.

This can be changed, via the Watch app on the paired iPhone, to 2 GB, 500 MB, or 100 MB, as preferred.

If you prefer to limit the playlist size by number of songs, it can instead be set to 15, 50, 125 or 250 songs.

The same size limits apply across all models of Apple Watch (including first generation, Series 1 and Series 2).

More information:

Listen to music on your Apple Watch - Apple Support