I don't think planting them "in clumps" is the answer. It's just one of those things that keeps gardening endlessly interesting-- every year is different. Where I am, two years ago was a drought year, it was hot and dry all season. I watered and watered but it was just too hot for the tomatoes, they did not do well. Last year was the opposite, wet, wet, wet. We got all the water from the year before, plus all our regular water. It felt like the year of no summer... cool, cloudy, rainy all summer. A better year for the tomatoes but all that constant rain and no drying out was conducive to diseases, which I had to fight. And hardly a good pool day in the lot! This year so far has been gardener's dream year, just gorgeous, enough rain but not too much. .. Hopefully the tomatoes will love it just as much.
Along with the vagaries of weather, the soil is a living thing and changes too. If you planted the tomatoes in the same spot the second year as the first (not a good plan if you can help it) and didn't add more soil fertility, the soil would be more depleted the second time and the tomatoes would suffer, being heavy feeders.
So how many to plant is always a bit of a guessing game. Over time you figure out what the average is and go with that. In a good year you will always have too many tomatoes (unless you like canning, in which case there is no such thing
); in a bad year there won't be enough. Oh well...