Source (news article): Russia to Start Bombing in Syria ASAP
Russia has also sent 16 helicopters, two surface-to-air missile batteries, nine T-90 tanks, and enough modular housing to hold 2,000 Russian troops, up by 500 in just a matter of hours. U.S. officials believe there are at least 500 Russian troops on the ground now, presumably to serve as advisers to Syrian forces on the front lines and help them launch more precise artillery strikes and support their ground forces from the air.
I'm not really sure that I am following that particular sentence. The line up by 500 throws me off completely. What exactly does it say? And break it down for me in terms of grammar too, please.