Return the current capacity of the registry of the logger manager singleton. A capacity of 0 implies no limit will be imposed; otherwise, new categories may be added only if numCategories() < maxNumCategories(). The behavior is undefined unless the logger manager singleton has been initialized and is not in the process of being shut down. Note that 0 < maxNumCategories() < numCategories() is a valid state, implying no new categories may be added.