In the ideal world the reallocate count will be zero.
In the real world it's not uncommon to see some value in that field.
Keep in mind the higher the value the slower your hd is getting.
How high is too high?........unknown.
If that hd is under warranty I'd contact the maker and have a chat.
With a little luck you might get a free swap out.
i ran crystal disk info(portable version) (why? this thread)
and my main hdd showed some intersting results i've been monitoring and the re-allocation count isn't going up but is firm at 100.
there's more system info in the linked thread(windows 7 backup failure)
saved it all to a text file too long for a post