Hello Isabella,
At the moment there's no straightforward way to access the number of key points for certain camera. We will consider such feature in Python API in future releases, though.
Possible approach is to analyze the start of the image matching log, where each line corresponds to the camera.key value:
2020-09-11 11:34:32 Matching photos...
2020-09-11 11:34:32 Detecting points...
2020-09-11 11:34:32 Found 1 GPUs in 0 sec (CUDA: 0 sec, OpenCL: 0 sec)
2020-09-11 11:34:32 Using device: GeForce GTX 980 Ti, 22 compute units, free memory: 2219/6144 MB, compute capability 5.2
2020-09-11 11:34:32 driver/runtime CUDA: 11000/8000
2020-09-11 11:34:32 max work group size 1024
2020-09-11 11:34:32 max work item sizes [1024, 1024, 64]
2020-09-11 11:34:33 [GPU] photo 6: 5624 points
2020-09-11 11:34:33 [GPU] photo 8: 5456 points
2020-09-11 11:34:34 [GPU] photo 3: 5524 points
2020-09-11 11:34:34 [GPU] photo 9: 5524 points
2020-09-11 11:34:34 [GPU] photo 10: 5626 points
2020-09-11 11:34:35 [GPU] photo 5: 5097 points
2020-09-11 11:34:35 [GPU] photo 11: 5097 points