I guess it's an interesting problem to have. Stockholm has three 'lines' that run through their downtown core. These each split into thirteen distinct destinations. Travelling in only the core, there are plenty of trains to catch, so long as you stay aware of the ultimate destination, and whether the train you hop on will be useful to you. I did experience a station with four lines inside, on two platforms. This was a transfer from regional to local trains (which we should have done for the airport platforms)
They also have a separate airport train, but it's super far to the airport, and the train is direct to the centre.
Following this logic, we still have just three lines. Line 1 will have three possible destinations. If the airport sh*t shuttle interlined at Southkeys, and the O-Train line were to be interlined at Bayview and head east through the tunnel before heading straight at the Rideau curve heading down Montreal Road (Wouldn't that be grand) we'd still have just two lines, despite the fact that they both use the downtown tunnel, and there are six destinations.
Just one more sh*tpost, I wanted to bring up Bangkok's BTS, they interestingly have no split ends. However, where lines intersect at transfer stations, the lines weave so that the most-used transfers involve a simple walk across the platform, instead of finding another station or platform. its insane to leave one train, walk 10 feet directly onto a transfer train, especially with several hundred of your fellow travellers. So efficient.