I don't know if it is possible to make that kind of call this early.
Attrition from 2 COVID-interrupted seasons could be significant for a lot of clubs; people who haven't been able to travel for a couple of years, with vaccination potentially opening up new doors.
But for a moment, let's entertain the possibility 2 clubs could go up from 2nd Division - Hunters Hill and St Pats being the obvious candidates. We need to cascade what that looks like for other divisions.
You're now left with 6 clubs in Div 2 - well except Old Barker are unlikely to stay, so now you're down to 5. Let's also assume the SS club going into Div 2 is Penrith - the same Penrith who struggled to put out 2 teams this year.

Having them field 4 Grades + Colts in 2nd Div seems like a stretch ... but OK we're back to 6 clubs in Div 2.
Where are the other 2 clubs coming from to make 8? Iggies and Beecroft? Count out everyone else in Div 3, but even then I'm not sure either club wants this.
If it happened, Div 3 is down to 6 clubs, and HAC and Blacktown are hanging on by the skin of their teeth. So does Div 3 just disappear for 2021? Div 4 might benefit in terms of numbers, of course...
Seems like a mountain to climb, to me.