Supervisa is used when they enter into Canada which would allow them a longer stay for that entry (vs normal vistor visa which is max 6 months).
When they extend their stay, it's just extending their visitor status. So I think you can request up to 2 years but if IRCC will give them 2 years or shorter extension, it's up to IRCC.
- People who currently have a Super Visa also have the option to request to extend their stay by up to two years while in Canada. This means current Super Visa holders will now be able to stay in Canada for up to seven consecutive years.