In Harry Potter and the Half Blood Prince, it was mentioned that the Ministry for Magic offered protection to Hogwarts by placing more counter-curses, anti-intruder jinxes and a small task force on the grounds of Hogwarts Castle for the sole purpose of protecting the school. So, how many Aurors did Rufus Scrimgeour place in Hogwarts grounds to protect the castle?
Tonks mentions herself and 3 others.
During Harry and Tonks' dialogue in HPB she mentions other Aurors guarding Hogwarts.
"But what are you doing here, anyway?" Harry asked
"I'm stationed in Hogsmeade now, to give the school extra protection," said Tonks.
"Is it just you who's stationed up here, or -?"
"No, Proudfoot, Savage, and Dawlish are here too."
"Dawlish, that Auror Dumbledore attacked last year?"
- Harry Potter and the Half Blood prince, Chapter 8 (Snape Victorious)
In the harry potter and the half blood prince game there are 5 or 6 Aurora on the marauders map guarding each area of Hogwarts, I can't find the particular map I want but if you play that game it will show you but this wikia shows you the locations which they guard, it may or may not be canonical to the movie though.