I've been processing deer leg sinew and have it separated into different lengths.

When I go to apply it to my bow how many strands should be in each bundle?

As many as you feel comfortable with and can lay them down smoothly. For me it was a learning experience as I went along. I have only made a few sinew backed bows!
 When you begin to back a bow with the sinew have a bowl of warm water and a damp towel handy. Makes things lots easier if you can work without sticky hands.

I like my bundles to be just about the size of a pencil.

Dan

I just put the whole pile in a bowl of warm water and pull a few strands out at a time.  Sticky hands and dry sinew make the job funnier for the standers-by but not for the one doing the work  

if you want to see how the PROS work sinew, go to youtube and search "korean horn bow".  Do yourself a favour and see how it's done right.