From now where we are currently stand:
Conservative prediction - 25 slams (Just one more AO title)
Realistic prediction - 27 slams (One more AO and two more Wimbledons)
Optimistic prediction - 30 slams (Two more AO, two more Wimbledons, one more USO and one more FO titles)
Wishful thinking - 32+ slams (Two more AO, two more FO, two more USO and 3 more Wimbledons = 5 career slams, shared all-time record at the USO and the positive W-L in finals at 7-6, all-time intergender record of 12 AO titles and close to Nadal's 14 FO and finally double digits at two different grand slam tournaments 12 at AO and 10 at the Wimbledon and intergender records at both! 12-10-7-5 is a personal wet dream of mine!)
A little more "down to earth" wishful thinking - 30 slams (Two more AO, three more Wimbledons and one more FO = 12-10-4-4 aka double digits at two different slams with intergender title records and four career slams). Why i think this is "down to earth" is because Wimbledon technically is the kind of grand slam that Novak will still have chance to be winning even in his 40's, because it's not nearly as physical as the rest of them and with the right scheduling and attitude it's possible for Novak to bag 3 in a row for the next three years, not saying that this is what gonna happen, but it's certainly not outside realms of possibilities and two more AOs is well easy peasy for Novak i wouldn't even be surprised if he ended going even beyond that at the AO, but two more in 2024 and year later in 2025 is certainly do-able, Wimbledon though is way trickier, but we'll see and 4th FO well...if he doesn't win it next season it's over, but if he does than all of this is useless polemics anyway...