What is the maximum solar PV amount that can be connected to a SMA SunnyIsland system?

For each watt of continuous AC output power, you can AC couple up to two watts of solar inverter capacity.

E.g. A split-phase system of two SunnyIslands has approximately 12kW cont. Therefore up to 24kWac of solar inverter capacity can be AC coupled.

This would be e.g. 4pcs SMA SunnyBoy SB6.0-1SP-US-41. You can also DC:AC oversize the inverter to accommodate a DC size higher than 24kW.

With a DC:AC ratio that is common and recommended for larger Off-Grid systems of approx. 1.5:1 you could go to ~36kWdc on the inital 2x SI6048.