How to count the number of selections in multiselect picklist?

How to count the number of selections in multiselect picklist?

Hello!
We have a form (see it live here under Step 2) where the non-prisoner public can select from our list of potential prisoner penpals and give us their email address.  (If you use it, please call yourself Test, test@test.com, and only select the Test prisoner records). 

Currently, we have an email sent whenever a form is submitted to people who review the requested penpals, and only approve it if there are fewer than 5 penpals requested. Ideally, there could be an automatic cutoff built-in.  Also, we'd like to be able to know how many non-prisoner penpals each prisoner has. 

Our structure:

Prisoner_Profiles form contains all of the demographic information about the prisoner penpal
Pen_Pal_Pairs1 form is the one linked to here; contains the information submitted about the non-prisoner penpal. 

There is a bi-directional link between the two forms via  multi-select lookups:
      Pen_Pal_Pairs1 is filled out first:
             prisoner lookup field is called: Your_Prisoner_Pen_Pal3
             value stored is: Mailing_Address_Date_Current
      On Prisoner_Profiles, the 
            non-prisoner penpal lookup field is called: Your_Email1
            value stored is: Your_Email
     
First request:
How can we fill a numerical variable "PrisonerCount" in the Pen_Pal_Pairs1 with the count of selections (Mailing_Address_Date_Current)  in the Your_Prisoner_Pen_Pal3 field as they are being selected? (I imagine this would be useful to people for a wide variety of reasons).  This action needs to happen before a post-submission email is sent.

Second request:
Different people who submit the form Pen_Pal_Pairs1 may select the same record (Mailing_Address_Date_Current) on Prisoner_Profiles. Therefore, in the Your_Email1 field in a view of Prisoner_Profiles, there maybe multiple non-prisoner penpals recorded.
How can we fill in a numerical variable "PenPalCount" in the Prisoner_Profiles that counts the number of non-prisoner penpals (Your_Email)? It needs to increase the PenPalCount as new Pen_Pal_Pairs1 forms are submitted adding new records.

I've looked at the instructions for Aggregate Records but it's unclear how to implement that. I saw this post, but it's a bit complex and I'm not sure exactly how to implement it for my case. Like where does that code go? In the form definition? In a configuration for a specific field?

Thank you thank you! This will be a big big time saver.