The limits to my different collections are varied and quite arbitrary. There are basically five areas that I actively collect at the present time:
1. U.S. postage stamps. I'm pretty much just filling spaces in my album with mint never hinged examples, when I can afford them. I almost never bother with used U.S. stamps anymore, though I did buy #599A and #634A as placeholders last year. I'm not buying any revenues, duck stamps, postal stationery, etc., and I cut off the collection at 2005 — though I'll still save a pane or booklet of a new issue if it appeals to me topically (see item #5).
2. Stamps and postal history featuring Esperanto and other artificial international languages. The limits of the collection are well defined as far as stamps and postal stationery are concerned: if there's Esperanto on it and I don't already have it, I need it. That doesn't mean I can
afford it, and you might be surprised how much is out there (see the vast amount of postal stationery from the USSR with Esperanto on it, for example). Cinderellas pertaining to these languages are also fair game, as are covers and postcards written in these languages, which makes the collection virtually limitless. I can't hope to get more than the tiniest fraction of what's available, so price and taste/fancy/whim dictate what I buy.
3. Stamps with runes or runic monuments on them. A very limited collection, and I already have all the stamps of which I'm aware (MNH, except for Iceland #162, which is MLH). I'm supplementing the collection with things like FDCs, when I can get them inexpensively. And if I learn of the existence of any more thematically relevant stamps, I'll buy them.
4. Stamps honoring J.R.R. Tolkien and his works, as well as FDCs of same. The British issues are the most important: 1992 Prestige Booklet, 1998 Magical Worlds, 2004
The Lord of the Rings 50th Anniversary. I've limited the Peter Jackson movie stuff to the New Zealand issues; just having those is already too much.
5. Other stamps or covers that interest me topically and/or appeal to me aesthetically. No guidelines here, just pure subjectivity: Mesrop Mashtots, Struwwelpeter, Osamu Tezuka, Monty Python, Elle Macpherson, etc. etc. etc. "Korea issued a stamp in honor of the 500th anniversary of
Hangul? I'll buy that!"