The number of possible 12 word phrases (disregarding the checksum) is 2048 ^ 12 = 106338239662793269832304564822427566085444517870735015415413993718908291383296
The number of alphabetically ordered 12 word phrases is
= 11005261717918037175659349191168
The second number is smaller by a factor of 500 million or a reduction of 99.9999998% but you measure such large numbers in bits in which case you get an entropy reduction of 29 bits (132 - 103)