I found 41. I was getting quite a lot of them originally, but then I was like "Walking through every square metre of each level and actively searching for these things is ruining the pacing" so I decided to play normally from about chapter 7 onwards, and to get the rest in a second playthrough.
65 on the first playthrough. I'm up to chapter 14 on my second playthrough now and I think I have around 85 - some of them were in places I was sure I had checked the first time around!! I guess they're just tricky to spot!