My one question/concern here, if anyone would be a

My one question/concern here, if anyone would be able to answer, is what is going to happen if they decide to do a buy back to the 400m range let's say, and those of us holding millions of shares decide to sell? I imagine at some point we are all going to want to sell, but at what cost to the stock/company? Won't it have a huge negative impact on the price? Would we even be able to sell so many shares at once? I apologize in advance, since I am still learning..thank you to anyone with a response.