On average you will need 50-120 liters pots , you can use a mixture of 50% garden soil & 50% potting soil.
50-120+ liters for 1 tomato
80+ liters for 1 cucumber
120+ liters for 1 watermelon
1 plant per pot
if anyone is interested in a few tips about plants in pots you can check this topic:
Optimize Your Garden Here - Topic - d2jsp
https://forums.d2jsp.org/topic.php?t=98498142&f=90&p=657277627As you can see on this picture i used 3x 38L pots for my tomatoes, but these are airpots, it's better to take bigger pots because more soil and so more nutrients.
https://i.ibb.co/YTZ2f9w/20230917-145548.jpg