How can I tell how many / which Students have Committed a Lesson within Lt?

There are several ways see how many Students have Committed a Lesson within Lt.
You can find out how many Students have Committed a given Lesson by going to Student Work | Lesson | Grading. The Grading Card will indicate how many total Students have Committed the Lesson:
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Note
: A Lesson is considered Committed if the Student has clicked the 'Commit' button on the Completion Page (kuraCloud Grading) or clicked the 'Send Email' button on the Completion Page (PDF Grading)
For a breakdown of which specific Students have Committed a Lesson, your options will vary depending on which Grading Mode you have configured:
kuraCloud Grading
If kuraCloud Grading is being used, the Lesson in question will need to have at least 1 Panel within the Lesson set to 'Graded' OR 'Graded + Instant Feedback', to allow access to the Grading Interface. Then, go to Student Work | Lesson | Grading, and click 'Grade Lesson' OR 'Grade Revision'. The Grading Interface shows only Students who have Committed the Lesson:
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PDF Grading

If PDF Grading is being used, the email address entered for the Course under Settings | Grading Workflow will receive copies of all Committed Student Reports, allowing you to easily search within your Email Client to find Students that have committed their work.