NEW DELHI (Reuters) -Gautam Adani faces a critical day on Monday with his flagship company's $2.5 billion share sale's second day of bidding overshadowed by a $48 billion rout in the Indian billionaire's stocks which was sparked by a U.S. short seller's report. Seven listed companies belonging to the Adani conglomerate, which is led by Asia's richest man, saw sharp falls in their values after Hindenburg Research report last week flagged concerns about high debt levels and the use of tax havens. Adani Group issued a detailed response late on Sunday, saying it complies with all local laws and had made necessary regulatory disclosures.