Step 1: Set Your Budget and, therefore, your number of photos.
Let's say that you've budgeted $300.00 for your photo video production (not including extra copies).  That means that you can have 150 photos and other scanable items.  $100.00 would be 50 photos, $500.00 would be 250 photos, and so on.

Step 2: Group your photos by song.
Since a standard song will hold between 35 to 45 pictures, using our example of 150 photos you should pick four songs.  This isn't an exact science, so don't worry about trying to figure out exactly how many pictures to put with each song.  Between 35 and 45 should do just fine.

Example:
"Brown Eyed Girl" - Smith001 through Smith042 (pictures of a little girl; the Bride, for example)
"Let Them Be Little" - Smith043 through Smith078 (pictures of a little boy; the Groom)
"You Raise Me Up" - Smith079 through Smith136 (Family pictures for both.  This happens to be a  4:50 song, so it will fit more pictures.)
"I'll Be" - Smith137 through Smith150 (pictures of the couple together.  In this example there are only 13 pictures for this section.  This is fine since it is the last section, a slower song and pictures of the couple together.  We would simply hold the pictures longer to fill the song.)