We don't have canon info about the size of a legion, but in [legends][1] it is roughly around 10 000 men, similar to a division in large number of real armies here on Earth, especially to WW2 divisions, and we know Lucas did get a lot of inspiration from WW2. Vader had approximately one battalion with him in [attack on Jedi Temple][2], which suggests that 501st Legion was split up, again something that does happen in real life, assuming that one [battalion][3] in SW universe has similar size to real life battalions (around 1000 men ) .
 
     Moving whole elite 501st Legion to Coruscant without apparent reason would probably raise suspicion. But moving one or two battalions in disguise of routine rotation, rest and replenishment would go undetected. Palpatine probably pre-calculated how much troops would be necessary for planned attack, so he acted accordingly. 


  [1]: http://starwars.wikia.com/wiki/Legion/Legends
  [2]: http://starwars.wikia.com/wiki/Attack_on_the_Jedi_Temple
  [3]: http://starwars.wikia.com/wiki/Battalion/Legends