I've voted for three, although it would be fair to say that this has evolved over the years. Dependent on the location/destination I will select a "main" camera & support system. This may be based around my M7, or a DSLR such as my E-1 or E-400. The bulk of the lenses I take will then be in support of the first choice. If I am going to a city, such as Venice, the M will be the first choice. For somewhere that I may need telephoto or fast AF capabilities, the DSLR takes pole position.
One may act as backup to the other, but then only one lens is brought for the backup. If the DSLR is first choice then I may, as in Vietnam earlier this year, take my IIIc.
Third choice is always a small compact, either film or digital. Digital is handy for short video clips, as has already been pointed out.
If I have to travel really light, then it's one main camera (M or Screw or E-400) with one or two lenses, plus a "serious" compact such as the G7.
As with other respondents I would tend to only carry what I plan to use on the day when on location - the rest will stay secure in the hotel.
Regards,
Bill