"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?"
2 Answers
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.
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$\begingroup$ I think I should make the third harder. Alright, just wait for it! $\endgroup$ Jan 31, 2018 at 10:29
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 '