Setting a value in a node when node is updated

Hello,
I am trying to do the below:

I have a node which contains a date field (attached pic of dsm output for the node. Look at "field_offered_date".
Now i want to create a rule that will get the size of the array for this field (its 25 in the pic) and assign this value to another new variable.
Is there a way to do this using rules?
Thanks