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	Just to add more grist to this mill-- music is written in 2/4-- 3/4 4/4 and sometimes 6/8-- if you wish to count-- learn that a forward and back basic ( salsa or mambo ) should be counted-- 1-2-3-4 and 2-2-3-4 signifing two bars of music-- as are all steps and groups of steps-- this mis conception comes from people watching too many stage choreographers counting down the beats to the next acsending bar-- all routine s are timed at bars per minute-- the only possible way you can have accurate beginnings and endings to sequences taking into account pick up bars-- if you find this a little too complex then get together with someone who really understands music
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The reason that we count 1,2,3,4, 5,6,7,8 is because the clave rhythm which Salsa is based on has 8 beats to it. The forward and back basic step is tied into this rhythm, so counting in this way emphasises that the 1 and the 5 are different. A dancer should be able to hear just one bar of Salsa and know whether that bar starts with the "1" or the "5". They should also be able to know when two "1" bars occur straight after each other (i.e. the music goes 1,2,3,4, 1,2,3,4, 5,6,7,8) and change their footwork accordingly.

The reason we count 1,2,3,4, 5,6,7,8 is to reinforce the link between the clave rhythm and our footwork. It is the clave we are actually dancing to rather than the beats of the bar (although obviously the 4/4 bar structure is aligned on top of the clave, so it is easy for dancers, and even teachers, to miss this subtle distinction).