One bumper plate.
Troy has made this with a very snug hole sizing, 50.3mm to fit over a 50mm sleeve, made to fit high quality bars perfectly. Due to the tight tolerance, only these Troy bar models are guaranteed to work: AOB-2000T, AOB-1500B, AOB-1500T, GOB-1800, GOB-800. Other brands with a precise 50mm sleeve will also work. Some cheaper bars are made with too wide a tolerance in sleeve thickness and won't reliably take these plates.
You will need competition bumpers if you're lifting over 400 lbs or so. The large steel hub is heavier than rubber, so the plate ends up being thinner than a solid rubber bumper. Less rubber also means the bounce will be lower and more controlled.
On the downside, comp bumpers are not good for high-rep workouts with low weight (less than a 35lb bumper on each side). They are thin and can get damaged. So if you're unsure, get economy bumpers.
|25 lb||17.72" (450mm)||1.28" (32.5mm)|
|35 lb||17.72" (450mm)||1.69" (42.9mm)|
|45 lb||17.72" (450mm)||2.15" (54.6mm)|
|55 lb||17.72" (450mm)||2.62" (66.6m)|
These Troy competition bumpers are at the Motion Fitness & Performance Center:
|Fit||Snug (about 51mm / 2")|
|Diameter||450mm / 17.72"|