Eggplants will grow in 5 gallon buckets but in my experience they'll grow bigger in a bigger pot. I used to grow 3 in a 3 cu. ft. container. Last year I went with two eggplants in a 3 cu. ft. container and those two produced as much eggplants as when I grew 3. This year I'm testing a 3 cu. ft. container with only one eggplant. A 5 gallon bucket is about 3/4 cu. ft. They'll grow without stakes. I use a small wire hoop tomato cage for mine because sometimes the eggplants can get heavy on the limbs and it helps keep the plant from blowing over in heavy storms.

I just planted two eggplants in a 20 gallon hydro farm grow bag. I put one of those tomato cages that form a triangle. I'm hoping it'll be big enough.
Sharon

I grow almost exclusively in EarthBox and DIY EarthBox (self watering containers AKA SWC). I put only one eggplant in each EarthBox even though the company tells me 2 would work fine. I get healthier plants when the plants are not crowded. I also put only one eggplant in an 18 gallon rope bucket container. My eggplants are Rosa and Rosita and they require cage or stake when fruit are present because the fruit pull the plant down - in my experience.

DL