Do I Need Multiple Tomato Plants?

One, alone, will do it.
"Plan B" is always have another one ready just in case the original one falls over in the wind or some other weather crisis happens. Here's a tip: since tomato plants do best in acidic soil, and coffee grounds are acidic----save coffee grounds you have to dump on the top of the soil. Good luck and dream of thick slices on top of cream cheese and you favorite bagel.

Broney
No, Tomato plants flowers have both male and female parts, 1 will do you find, I make sure I always plant 2 also just in case something goes wrong with 1 of them.
I do suggest that you go out and gently shake the plant twice a day while blooming, with the Bee population being as bad as it is you may not get enough activity to insure a good crop.
Babbie

I always keep seeds from any tomatoes I eat or chop and throw them out in the garden. They grow nicely. The more the merrier! Robyn

Plant some basil with it, to help prevent insects.

You only need one for fruit. But having a back-up in case one doesn't make it for some reason is also a good idea.

One will do nicely.