Saturday night's "Comic Relief '87" raised an estimated $2 million during the 4-hour, 10-minute benefit telethon for the homeless. Home Box Office Inc., which produced and broadcast the show, plans to announce the final tally on donations sometime today. More than 50 comedians performed free before the capacity crowd of 6,000 at the Universal Amphitheater. The show, which HBO broadcast free (to non-subscribing cable-TV households), had a potential audience nationwide of about 40 million. The network had no estimate Sunday on the number of homes that actually tuned in to the program.