The minimum is whatever you want it to be: 

Delegating 1SP will give you a 10SP upvote 
Delegating 123SP will give you a 1230SP upvote 
Delegating 1000SP will give you a 10000SP upvote 
Delegating 2000SP will give you a 20000SP upvote 

*To see the potential upvote type the amount in steemnow and see what you will get.*

We want to give people the freedom to delegate what they can. 

Upvotes will also give more.