Heh, I had to go through my gallery and figure this out.
01/07 - 5
02/07 - 8
03/07 - 4
04/07 - 1
05/07 - 3
06/07 - 5
07/07 - 37
08/07 - 4
09/07 - 14
10/07 - 2
11/07 - 15
12/07 - 4
01/08 - 0
2007 total: 102 (many of which cost 5-15 dollars)
This count includes gifts and is based off purchase date (one of the ones I "acquired" in Sept. is still sitting at my deputy).
Other notes of interest:
1. In mid April '07, I went on a one month cel hiatus. After the break, I found it difficult to justify buying the same kind of pieces I did before--which was probably good for my wallet.
2. In July of '07, I bought a humongous set of Escaflowne cels/sketches for pretty cheap.
3. In September and November '07, I did my first two split auctions, getting several nice One Piece sketches.
4. January '08 may very well be the first month since I started collecting that I haven't bought anything.

Though I did win something this morning.
So, yeah . . . any time there's a number over 8, that was a month in which I bought sketch sets.

I bought cels in lots in some of the other large months.