Yep, the rules of golf state that the same brand and model of golf ball be used for an entire round. See the next post ↓ ↓ ↓
Pros generally only play a ball for 3 holes. Some rotate through the 3 balls in a sleeve playing one ball on every 3rd hole, like the 1st, 4th, 7th, 10th, 13th, and 16th holes
One ball gets played on the 2nd, 5th, 8th, 11th, 14th, 17th holes, and the 3rd ball in the sleeve gets played on the 3rd, 6th, 9th, 12th, 15th, 18th holes.
If you join the USGA ($10) you'll get an official 2013 US Open hat, and a copy of the Rules of Golf. Here is the link - www.usga.org