One SpriteFont with instance variable.
As was stated, you will have to have multiple instances of your single SriteFont object. The number of instances would vary depending on needs.
Just a quick example:
4 instances of said SpriteFont with different variable values for each instance.
SpriteFont | is on screen
Variable = 1 | set text to "this is instance 1"
Variable = 2 | set text to "this is instance 2"
Variable = 3 | set text to "this is instance 3"
Variable = 4 | set text to "this is instance 4"