Your title says

> ... that is iterating through each line of a file ...

your post says

> ... trying to take the first 16 lines of a single file ...

that's confusing.

I see three possible cases:

• first chunk of 16 lines
• successive chunks of 16 lines
• overlapping chunks of 16 lines

The second example in the following post shows an approach to generically solve all cases with just minor changes:

Re^2: Grab 3 lines before and 2 after each regex hit (sliding window)

HTH!