What is better 1 bigger sub or 2 small ones

As with most things...it depends.

Good larger subs generally go lower with flatter frequency response. A single sub will sound more different in different parts of the room in many cases, so there is more of a sweet spot. Two subs enable more balanced response in more locations within the room concurrently.

Also the right sub depends on what speakers used with. To fill in the low end effectively with speakers that are more full range already, you will need at least one large sub.

So to get the best of everything in theory you want two larger subs. Good ones two.

If cost is a factor, then you have to determine what compromises you are OK with and decide accordingly.

multiple subs are usually introduced in a large room for HT ..... Not for 2 channel

Ridiculous statement.

Agree with the, "it depends", viewpoints. However, do not forget that subwoofers are speakers too. So do not percieve or treat them with any less consideration than your main speakers.

Multiple subs are harder to set up properly

I also disagree with this statement. I agree that it is possibly more difficult. But not necessarily. Once again, it depends. For example, if the room is such that placement of a single sub disallows it to ever be properly set up, that is, no matter where you place it, you have noticeable issues (i.e., frequency suck-outs and/or boominess), then a second or multiple subs might be used to eliminate those issues which a single sub cannot. In this case it is then easier with multiple subs to "properly" set up.

According to the book, "Get Better Sound", 2 are required for accurate imaging, and all the old saws about how bass is non-directional are beside the point.