Four trusses on an outdoor plant and six in an unheated greenhouse is a reasonable expectation. You should pinch out the growing point at the top of the stem once it's formed these to force it to put all its effort into growing and ripening the existing fruit. Beware of sideshoots though!
If the plant is healthy and well-fed, each truss should produce more fruit. Varieties differ in their productivity too. Other than that, if you want more fruit, grow more plants!
Much depends on when you start the toms..........too early and the plants get "drawn up" and the first truss will be higher up the plant. In a decent summer, 6 trusses on outside plants to ripen on the vine is feasible. Personally, I'd be disappointed with only 4 but as Steve says it can also depend on the variety
very much agree with philippa. Some years ago I started suncherry off in my heated propagator in the frost-free g/house beginning of Feb. so probs with heat but the plants got so lanky. I planted 1 in the g/house just to see what happened - 1st truss was about 4' up the very thin stem.
So light is key to getting a short internode spacing and likely more trusses, but as also said above too many trusses and you may end up with a lot of potential chutney material!
...and I think they get lanky if they're too warm too soon.
Hmm, my dad told me 3 outdoors or 4 inside - I probably have 3 or 4 in flower on most but I suspect there are buds for other trusses above that I've missed when counting. I'm also thinking it's getting a bit late in Scotland so I should pinch them all out soon anyway....
If they go a bit fast at the start before setting their first truss, I just pot them on extra deep. They're quite tolerant of being bent a bit when being planted.