How do you determine the number of shares in a company?

None of these have one-size-fits-all kinds of answers - these are all complex, so here is some brief, high-level info. There are books such as Venture Deals (by Brad Feldt) and other handbooks readily available online on these topics.

While typically startups start with a nice round number of 10M shares of common stock authorized, with a majority going to the founders, it will depend on the number of founders, their dynamics, experience, intellectual property involved, etc.
At formation, startups often set up a stock option pool size between 1M and 2M shares of common stock reserved for issuance. This will depend on many factors though, including key employees, contractors, business model, etc.
While a startup can set a valuation cap on a SAFE, it generally does not directly determine what amount a SAFE converts at - this will depend on the amount of money being raised during a priced round, the exact (and custom) terms, etc. YCombinator has a webinar regarding this topic.
Please see here regarding raising and post priced round dilution: https://www.decilehub.com/base/1-general_questions/21536-what-is-the-optimal-ownership-percentage-for-founders-post-series-a

How do you determine the number of shares in a company?

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