How many specialized catalogs should a world-wide collector or casual collector purchase?
Specialised catalogues are especially useful when a collector has, or intends to have, a sufficient number of those stamps pertaining to the area of study to warrant the purchase. Alternatively, there may be an interest in learning more about the subject matter, regardless of the number of such stamps in one's collection.
There is, of course, no definitive answer to the question of who should buy or how many should be bought.
In terms of probability, the 'world-wide collector' who aims for one of each stamp is unlikely to need specialist literature beyond the purchase of an excellent catalogue to identify their stamps.
The presence of a large number of numeral postmarks in any colonial New South Wales or Victoria collection may prompt the philatelist to invest in one of Hugh Freeman's excellent books so that rarity and other information may be ascertained.
The casual collector may decide to buy a bulk lot of Australian KGV stamps or low denomination kangaroos - in either case the purchase of the relevant Australian Commonwealth Specialists' Catalogue may well turn out to be a very astute purchase.
There are no guarantees as to what lies within any lot or collection - the thrill is in the hunt and it helps to know what to hunt for!