$625 is an approximate number based on Matt Levine's reporting. It's not a fixed number; it is based in part on how many shares Musk has leveraged to fund his lifestyle. As the number is from last year and Musk has sold some stock since then and leveraged more stock, it's very likely the number has changed but nobody has done an in-depth analysis recently.

Yes, as CEO of a publicly traded company there are limitations on his ability to pledge his shares of that company's stock, and that limit is far less than "number of shares held." This is not a legal requirement; it is a de facto requirement relating to eligibility for the S&P 500 and of corporate governance requirements by many public pension funds and other large investors to buy a company's stock.