Most of the time, it's two numbers in that column, so min:max or avg:max should fit. Since probabilities are likely to be two-digit numbers, three could fit, but 100:100:100 might start pushing it.
Yes, two digit may fit. I am thinking not about fitting but about whether most of the players will be confused by two numbers for mods where is only one such UFO?
One option would be to display the average. This will still be a single number suiting both single and multiple assaulting UFO mods.
For multiple UFOs it won't be very precise, though, as they can vary in wide enough ranges if their capacities are different enough. But what we can do if base defense made so variative by the mod itself?
So the options are:
1a) one number for max
1b) one number for average
2) min-max, although interface would be kind of complicated and not very clear for the same numbers range.
3) number is average but bar is min-max?
Well, there are also base assault missiles (UFOs that explode and take out facilities instead of assaulting) and instant base defences (objective 6), so you could display one of those instead of min/avg.
Yes, there are plenty of variations. I don't plan to cover them all. This is essentially a statistical calculator helper not pretending to be an ultimate answer to all questions.