Grammar – Capitalization Rules When a Sentence Begins with a Number

Unless this is a technical term I don't know about, launch is used for the moment when a product goes on the market and is effectively launched by the manufacturer. Otherwise spacecraft are launched into space. Hard to tell without knowing what kind of device.

since then in that sentence is awkward as hell, if not downright incorrect.

I would use thereafter or from thereon in or from that point forward (in order of personal preference).

No, it literally means 20 eyes, or 20 eyeballs – not 20 pairs of eyes. However, it's impossible to answer this question unless you assume

• that everyone at the library looked up at him,
• that everyone had two eyes, and
• that all of them were people (= persons).

Presuming all of the above, there were 11 people at the library in total that I count. This is still an approximation because the initial "20 eyes" sounds like an estimate in the original sentence merely by virtue of twenty being a round number.

To address your comment:

Rounding can be a form of estimation. 11 rounded to the nearest ten is 10, which would be a valid answer when asked for an approximation.