If all the necessary information about the company at the moment is already disclosed to the public, are insiders allowed to immediately sell as much as they want? Is there a limit?
There is no law preventing or limiting "insiders" from buying or selling shares, but officers, directors, and other key employees must report their trades to the SEC. What constitutes illegal insider trading is when the trader has material, nonpublic information about the security. This law applies not only to actual insiders, but to friends, family, or anyone who obtains the information. If an insider provides material information to someone who then trades based on that information, then both parties may be guilty of securities law violations.
That said, many companies have internal policies that prevent certain people from trading in order to prevent illegal trading, particularly around the disclosure of quarterly earnings statements and the disclosure of significant activities, as they can have a significant impact on stock prices.