That is a misleading or poorly worded statement, that statement indicates that OCS cannot handle any more people and that just isn't true, OCS has run at twice the capacity it is running at now. They can put through however many people the manning document allows them to commission per year. The manning document will indicate how many of each designator will commission per year, that info is taken and then it is figured out how many of each designator per class based on a few factors, one of which is when the follow on school will be. Some designators such as SWO, nuke and aviation have more flexibility since they have follow on class more frequently than other designators.
What is nearly always the case is the number of spots at OCS for the designator in question have already been booked out X number of months. This is why you will have people selected for one designator that get an OCS date 1 month out and others that get an OCS date that are many months out.
Prior to AD ISPP and ISEL the forecasting was a bit more accurate, those 2 programs have added some additional variables which appear to have caused things to be a bit more interesting.