“I can be what this puzzle is in” (2nd of a riddle series)

Question

"I can be some amount of time,
and I can be what this puzzle is in.
But not only this puzzle.
I can be used to look at 'yourself',
when it's bright enough.
But that's usually not the purpose I was produced.
When I'm 'many', most of you use me.
In a more literal sense, I can contain 'fourteen'.
What am I?"

Answer 1

You are probably

a window.

"I can be some amount of time,

We refer to "windows of time", the brief period of time when a thing is possible to do.

and I can be what this puzzle is in.

This puzzle appears in a browser window, for most of us.

But not only this puzzle.

There are lots of other windows open on my desktop, for sure.

I can be used to look at 'yourself', when it's bright enough.

In bright enough backlighting, a (physical) window can show you your reflection.

But that's usually not the purpose I was produced.

Windows were made to let light in and let you see out.

When I'm 'many', most of you use me.

"Many" being "Windows" ... 10, perhaps :)

In a more literal sense, I can contain 'fourteen'.

OP notes 'fourteen' references the atomic number for Silicon, a component in glass.

Answer 2

Are you

A quote?

I can be some amount of time,

Quotation marks can be used for writing a time (like in Usain Bolt ran the 100m in 9"58)

and I can be what this puzzle is in.

This puzzle is in a quote

But not only this puzzle.

The tilte also is in quotes, and many things can be.

I can be used to look at 'yourself', when it's bright enough.

?

But that's usually not the purpose I was produced.

?

When I'm 'many', most of you use me.

?

In a more literal sense, I can contain 'fourteen'.

'fourteen' is contained by two quotes '