How much tomato feed per plant?

That's something I've always wondered. Given that I've followed the instructions and put a capful of feed in 4.5 litres of water, how many tomato plants should that feed? And should it be replacing the normal watering or as well as?

It usually tells you on the bottle.

For example.....mine says 1 cap for 1.5 gallons which is sufficient for 9 plants in the greenhouse and 12 plants outdoors

I would always follow the instructions on your bottle of feed as all feeds will be slightly different, I personally fill a 6ltr can of water and use one capful of my feed (15ml), and that does 4 of my plants.
IMO however much water you give your plants daily, you simply add the feed to that amount and water as normal, though only using the feed once a week after the first truss has set then twice a week after the second truss has set.