CodeCommit: What would be the way to get change count for a Commit?

I would suggest using git commands to pull this information.

The git diff subcommand supports a --stat argument which will give you the summary of changes, with the number of changed files, along with the number of lines added, modified, or deleted.

Here are some examples, using the aws-cdk repo:

• Changes between most recent commit and previous commit:

$ git diff --stat HEAD^1..HEAD
 packages/@aws-cdk/aws-customerprofiles/package.json | 1 +
 packages/@aws-cdk/aws-groundstation/package.json    | 1 +
 packages/@aws-cdk/aws-lookoutmetrics/package.json   | 1 +
 3 files changed, 3 insertions(+)

• Changes between two tagged releases:

$ git diff --stat v2.68.0..v2.69.0
(I have omitted the first bit of the output here for brevity)
 packages/cdk-assets/test/docker-images.test.ts                                       |  139 +++++
 tools/@aws-cdk/eslint-plugin/lib/rules/invalid-cfn-imports.ts                        |    2 +-
 tools/@aws-cdk/individual-pkg-gen/transform-packages.ts                              |    6 +-
 tools/@aws-cdk/pkglint/lib/rules.ts                                                  |   24 -
 tools/@aws-cdk/prlint/test/module.test.ts                                            |    3 -
 version.v2.json                                                                      |    4 +-
 400 files changed, 10821 insertions(+), 7443 deletions(-)

As you can see, this gives you the total changes in summary, and also shows you a breakdown per file.

[1] https://git-scm.com/docs/git-diff#Documentation/git-diff.txt---statltwidthgtltname-widthgtltcountgt