Elon has about 170 million shares of Tesla. This sale will be 17 million shares.
Elon had 22.8 million stock options awarded to him from a 2012 CEO compensation plan. He met the objectives of the plan which included things like raising the value of the company by ten times.
The options must be exercised by August 2022 or they will expire worthless. Those exercised options will cause Elon to owe about $15 billion in taxes.
Sell 17 million in shares for $20-21 billion and pay $6 billion in taxes on long-term capital gains. Exercise the options and pay the $15 billion in taxes. Have nothing left over if the stock price does not move around too much. Elon will still have a net gain of 5.8 million more shares of Tesla after the options are converted and the stock was sold to pay taxes.