Need help with structure of DNA?

I am making a model for Bio cl of a simple DNA double helix molecule (with sugar, phosphate, 4 bases, and that stuff). I know that there are two strands that make up the double helix but what I dont understand is how long to make each strand. I don't mean the physical length like 6 feet or whatever. I mean like how many nucleotides make up one strand of dna?