I only know the salsa scene in my area, but I think it's perfectly acceptable to dance multiple dances in a row with your SO (although I know many salsa couples who don't seem to do that -- mainly because, it seems, they both enjoy dancing with different people). On the other hand, if you dance 3 dances or more back to back with the same partner, or are seen to be dancing with each other a lot more than with other people, then people will probably assume that you two are a couple and may even avoid asking you/your partner to dance for fear of offending you. This is the reason why I usually turn down a third dance (I've always thought two dances in a row is okay -- not quite sure why myself).
In the Latino community, much like the Europeans, it's customery to dance at least two songs in a row with someone you've just met, or not too familiar with.
More than two (to a stranger), it might be misinterpreted as interests more than a dance (therefore the guy may try to ask for your number etc afterwards). So for the followers, if you really enjoy dancing with the guy, but not interested in that way, thank him at the end of the second dance. You can always ask to dance with him again later in the night.
If you only do one dance it might be a bit on the rude side, back in the days. But nowadays (with the American influence I guess?), it's quite ok to do just one.
These days most people in the salsa scene generally assume that you will agree to dancing one song, with an optional second one if both are happy to continue.
I do three or four in a row with good friends, SOs, dance partners, people I havn't seen for ages, or my hand-picked protegees.
In terms of couples, there are two different modes: they dance exclusivele with each other (unless there's the odd ones with good friends/relatives), or they dance with everyone (no more than two in a row) with the exceptions to 'special' songs.
We got "hoggers" too, who disregard (or completely oblivious) all of the above; they dance songs after songs with one of the most popular leads. For some mysterious reasons guys actually let this happen more often than you think :x !!!!!
In a leader-short city like mine, you have noooooo idea how irritating this is!