From ufc.com rules and regulation:
UNIFIED RULES AND OTHER MMA REGULATIONS
A. All bouts will be evaluated and scored by 3 judges who shall evaluate the contest from different location around the ring/fighting area. The referee may not be one of the 3 judges.
B. The 10-Point Must System will be the standard system of scoring a bout. Under the 10-Point Must Scoring System, 10 points must be awarded to the winner of the round and 9 points or less must be awarded to the loser, except for a rare even round, which is scored (10-10).
C. Judges shall evaluate mixed martial arts techniques, such as effective striking, effective grappling, control of the ring/fighting area, effective aggressiveness and defense.
D. Evaluations shall be made in the order in which the techniques appear in (c) above, giving the most weight in scoring to effective striking, effective grappling, control of the fighting area and effective aggressiveness and defense.
J. The following objective scoring criteria shall be utilized by the judges when scoring a round:
i. a round is to be scored as a 10-10 round when both contestants appear to be fighting evenly and neither contestant shows clear dominance in a round;
ii.a round is to be scored as a 10-9 round when a contestant wins by a close margin, landing the greater number of effective legal strikes, grappling and other maneuvers;
iii. a round is to be scored as a 10-8 round when a contestant overwhelmingly dominates by striking or grappling in a round.
iv. a round is to be scored as a 10-7 round when a contestant totally dominates by striking or grappling in a round.
So, a judge decides winner of the round by using objective criteria mentioned in J (and See K also). And it seems 7 is the minimum point that loser of a round may get (see J. iv).