The company I work for has two of the HP R3000 XR UPS devices. We actually had an electrician come out and verify that everything was okay with our setup.
Each R3000 XR is rated at 30A and has six outlets, which are divided into three circuits by pair vertically. Each circuit is rated at 15A. The electrician said that you should not exceed 80% of the rating, though you are theoretically safe up to the maximum rating. This means that you should avoid anything over a total of 12A on each circuit. We had approximately six devices (servers, a robotic tape library, server rack fans, rack monitor, etc... mostly servers) plugged into two of the circuits on each UPS, with one circuit on each going unused. The UPS' were drawing 15A and 13.3A, respectively, on their 30A dedicated circuits. That is well under their rating and also well under the maxed advised draw of 24A.
It is reasonable to assume that you can split the draw per UPS in half (since there are six devices in each circuit), by circuit to determine the load per circuit. On UPS A, the draw per circuit is ~7.5A, and on UPS B, the draw per circuit is ~6.65A. Again the maximum recommended per circuit is 12A, therefore six devices per circuit is perfectly acceptable and leaves room for fluctuation, improper division, and some expansion.
In any case, it is highly advisable to split devices evenly as possible among all available UPS' and the circuits on the UPS'.