This depends very much upon what you are trying to achieve and there isn’t an answer that suits all occasions. It’s up to you to read the situation in front of you. Take into consideration what the weather is like, are they feeding, are there carp in front of you, how long have you got, what sort of bait are you using, are you trapping ‘em as they pass or are you encouraging ‘em to feed on your spot. All these questions have a bearing on how much you should be putting in and people generally say it’s best to put a small amount in first then build on it. I’d agree, but do you really know what you are putting in and what it looks like on the bottom? Bear in mind particularly when spodding, that you are not putting in anywhere near as much as you think you are.
A lot of your spod mix is liquid and mini particles that disperse in the water and never make it to the bottom. If you think half-a-dozen spods is enough, double it to achieve what you think you already have!
Accuracy will also have a massive effect because the chances of you getting everything in the same spot across a large number of casts are almost certainly nil. The chances are that you are not fishing a tight spot anyway!
Finally, throwing-sticked boilies might look like they are landing in the same place from the bank, but in the water it’s very different. Even a hundred baits scattered across a 20ft x 20ft area is not actually a bed of bait. A lot of the time we think we are fishing over a lot of bait when in reality it’s nowhere near what we think it is!