This is actually a collection of myths regarding search engine submissions. Among them you'll see talk of submitting to 15,000 search engines, 20,000, or any number of additional search engines. Let's clear up a few things.
First, you don't really NEED to submit to any search engine optimization to wind up on their indices. Of course, you'll most likely be there faster if you do. You'll be working on your schedule, not theirs. This also gets around another problem. The thing about waiting for the search engines to find you is that they have to find you by an inbound link. In many cases, getting an inbound link involves being on the search engine indices anyway.
There are, of course, exceptions to this. If you're familiar with Conversation Domination, you know that you don't have to wait for inbound links. You can build them yourself in Web 2.0 properties. In this case, the search engines will find you without you submitting the site to the search engines.
Now, about this talk of submitting to 15,000 or 20,000 or a million search engines. There are only three search engines that really matter to the average user. These are Google, Yahoo, and MSN. Although Yahoo and MSN are losing their importance to the Almighty Google, they're still important enough to go ahead and submit your site to.
The large bulk of the rest of the search engines don't really matter to the average user (read customer). Besides that, there's a little secret that these SEO guys who are selling this astronomical submission concept aren't telling. Want to know what it is?
The large majority of the other search engine optimization have this little graphic on them that says "Powered by Google," or "Powered by Yahoo," or "Powered by MSN." There may be one or two other search engine providers that power other search engines, but by and large, it's the big three. Now, what could that possibly mean?
It means that they draw their search engine optimization results directly from Google, Yahoo, MSN or whatever other search engine provider they're "powered by." You are, in effect, submitting to these sites by proxy when you submit to Google, Yahoo, and MSN. The only service they really provide is that they limit the scope of their searches to specific niches. That is, assuming that's what they're really doing. How are you to know, really?
On another note, are there really that many search engines out there? I haven't exactly taken count, but I somehow doubt it when it comes to some of these claims. Besides, if there are that many search engines, you've pretty well covered your bases by ranking on one of the big three. Anything outside of that is likely to be irrelevant.
The search engines that tend to be "outside the pale" of Google, Yahoo, and MSN tend to be interlibrary and university search engines. Unless you're conducting scientific experiments in marketing, do you really need to be on JSTOR?