Could anyone tell me what are the factors that determine the depth of a sequence?
That doesn't exist, but sequencing depth does, it's the same as coverage. Please use search first.
Before we slam the door on this question: Do you mean "how do I calculate coverage?", or "what biological and technical factors influence depth for a given gene in DNA-seq or RNA-seq?". The first question has been answered several times on the site, but the second question is more interesting and an an active research area.
Even if we can construct something interesting, I don't think that it is our job to guess what OP might have meant.
I had closed this as exact duplicate in Biostar1, listing two questions. A search on the Biostar site yields sufficient results: http://www.biostars.org/search/?q=sequencing+depth No way, we can justify to keep this question open.
What is sequencing depth