There have been two released so far, each compiling 5 Spotlight stories.
They often do compilations in lots of 6 with the various ongoing series, probably to match up with the 6-issue structure (and compilations) of their mini-series, so it sounds a little 'cheap' only getting 5 in the Spotlight compilations.
Anyway, there is a third Spotlight compilation that was just listed in Previews in July, which means it should be out some time in September.
Just looking it up, these are the Spotlights so far released:
(note - IDW didn't actually number the issues, so that it didn't put off people buying a current issue thinking it was part of a continueous series. Each ties into their other TFs series and mini-series, but can stand alone as a one-off story)
(Compilation 1 - Out May 2007)
1 - Shockwave
2 - Nightbeat
3 - Hot Rod
4 - Sixshot
5 - Ultra Magnus
(Compilation 2 - Out Feb 2008)
6 - Soundwave
7 - Kup
8 - Galvatron
9 - Optimus Prime
10 - Ramjet
(Compilation 3 - Sept 2008)
11 - Blaster
12 - Arcee
13 - Mirage
14 - Grimlock
15 - Wheelie
(Compilation 4? - may only comprise the 4 issues that make up the 'Revelations' story arc)
16 - Cyclonus
17 - Hardhead
18 - Doubledealer
19 - Sideswipe
(future known Spotlight, that might be part of the above 4 in their compilation.
20 - Blurr
Links below have spoilers:
http://en.wikipedia.org/wiki/The_Tra...ers:_Spotlight
http://transformers.wikia.com/wiki/Spotlight
http://en.wikipedia.org/wiki/The_Tra...rs:_Revelation