Title says it all. I need to grow ONE plant. How many seeds to grow? It's a small clay pot.
If you really need one plant growing in the center of the pot, then get 3 or 4 pots, mark them with numbers (so you don't bias your experiment by choosing the one that is "growing the best" or whatever) and plant just one seed in each pot. Use the lowest numbered pot where the seed germinates for your experiment You can eat the other radishes in a salad, of course!
If it doesn't matter whether the plant is in the center or not, put 3 or 4 seeds in one pot equally spaced apart, again mark the pot with numbers so you know which plant you will keep before they start to grow, then pull out and throw away all the plants except one.
Radish seeds should germinate in 4 to 14 days - if not, you have done something wrong!
If its radishes, and only one small pot, plant 3 or 4 seeds - thin out later after germination, when they've developed two true leaves each (the first 'leaves' are the cotyledons, and don't count) to leave the strongest growing one. By 'thin out' I mean remove them.
It's still safer to start off two pots though, then if anything happens to the strongest seedling, you've got a back up going already.
Seeds have different germination capacities among species, so it depends on what you are sowing. The germination capacity varies with seed age, so you will get more seedlings from last year's seeds than from older ones, let's say from three years ago.
Considering that for some species the time of the year when they are sown is really important, I would use at least 5 seeds from the beginning and then I would thin them out, instead of sowing one or two and then sowing some more at a later date in case the first ones didn't germinate.
Since all seeds may or may not germinate, I recommend you plant 4-5 seeds. I would sow them a few inches apart rather than together (I have done that in the past) so if more than one germinates (which is likely), you can keep the strongest and remove the rest easily by pulling them out. If you sow them together, pulling one out could damage others since the roots may be intertwined. If they are farther apart, you can pull them out or cut them from the stem so you don't damage roots of the healthy one.
I concur with Bamboo, but I might also add that radish seeds generally seem to have excellent germination rates (and they generally sprout fast). So, odds are high that if you just planted one seed per pot, you'd be okay (although I've never grown them in pots or anything but garden soil). However, it's still a risk. If you don't want to risk it, plant a few more seeds and thin, if it won't harm your experiment. If you don't want to select for the strongest, just thin random ones (not ones you specifically choose to thin).
Use seeds that say 'packed for 2017' (if you're buying them in the USA) for the best results. That means their germination rates have been tested and are high enough to be sold legally during 2017. Each state has its own laws on germination testing.
One option (as with all, depending on what you are seeking to find) is:
Germinate the seeds first, e.g. using paper towels kept constantly wet for a few days. Then just divide the seeds into germinated and not germinated (a binary choice will help avoid the 'selection of the fittest' effect mentioned by others) and plant them in pots, one germinated seed per pot.