Firstly you need to value the company according to market research and references. If there are other startups in the space and what they're valued at.
Distribution of shares happens while incorporation. Investment will be added after incorporation if I'm not wrong, not entirely sure.
SAFE can be risky for founders, better to have a valuation before hand. Otherwise investors can get a lot more than 20% depending on terms. Ideally you shouldn't give away more than 10-15% equity in the first round.
In the future rounds the existing shares will be diluted to make way for new shareholders. Or one of the existing holders can sell their stake. New shares are not created.