In this case the "Unlimited submissions" (i'm guessing you're using the File Submission type) relates to the number of "submissions into File Submission type", not the number of submissions / steps for an assignment.
Out of the box Moodle doesn't support this.
You may be able to "clean up" the site for students by looking at Restrict Access conditions that make each step visible or not to the students depending on whether they have "completed" a previous step ("completion" doesn't necessarily mean anything to do with grading).
You may also be able to hide the "previous steps" based on them being completed as well, so that they only see 1 step at a time.
If each "step" is graded, then you probably want to look at making sure they're all in the same grade book category, and then use aggregation to synthesise a score, or a calculation if it's more complicated, or:
- hide the grade items for each of the Assignments and award the grade at the category level
- -or- leave the "last" assignment's grade visible and award this as the "final" grade.