Which Republicans left the Democrat party and went to the GOP,  how many?

This is a perfect example of people "just knowing" things that simply did not happen.

"Then, those Democrats left the Democratic Party and became Republicans"  is a false statement.

Of all the Dixiecrats, a single Senator,  Strom Thurmond defected to the GOP.  
All of the other Dixiecrats not only retired as Democrats, they were given committee seats and high positions in the party.  A couple of examples are William Fullbright and Grand Kleagle Robert C. Byrd.

And I am also familiar with the myth about Nixon and the southern strategy.  Another cherished narrative. I have searched and searched, and the only reference I can find about Nixon is that he was a supporter of affirmative action.    

Also, if you look at the shift in voting demographics in the south from Dem to GOP, yes it did start in 1972 but the bulk of the defections of voters did not occur until the 1980's and 1990's

data vs. myth