There are 2 ways to see how many members enrolled on a specific program within a specific time frame.
The first way is to use the Program Verification Report.
Reports>Financial Reports>Program Verification
Then choose the specific program, your date range and choose "Date Program Was Signed" or "Date Program Starts".
The second way is to use the Statistics Report:
Reports>Statistics>Stats Overview by Date Range
Then enter your date range and click "Go". Towards the bottom of the report you will see a total number of programs.