wrote:
Mercury roughly goes around the sun 4 times for every one revoultion of
the earth round the sun, Does this mean that there are 4 transits every
year? if not why?
Thanks:

That would only be true if Earth, Mercury, and Sun were always in
perfect alignment, but they're not. Relative inclination of the orbits
play a big role here. Most often, Mercury would appear to pass above
or below the Sun as seen from Earth.