Spread? That's an understatement. One will be enough!
I have moved mine to a scruffy bank behind the shed where it is better than the nettles etc that were there before. It is a very pretty blue flowered one and the bees love it but it runs like its overdosed on senna...

I put in 3 and within a year they were taking over the plot they were in!!!! Just one will be enough, somewhere out of the way, not really fussy about soil, the leaves are great on the compost tho, or making liquid feed which absolutely stinks too :))

They seem happy to grow anywhere , OH has just harvested ours to make liquid feed , and it is currently fermenting ( apparently it will be rather smelly ) .
It will grow in any spare corner , I never planted ours , it just arrived , and like Sterie says it certainly spreads .
We have lots everywhere but the bees love it , ours is white , and it is a good filler until the garden takes shape in late Spring , Jen .

Hi
You want a plant that dose not set seed so go for Russian Comfrey Bocking 14 its a sterile plant so you can only propergate from the roots on ebay £1.99 and free p&p now if I can say that on here ? if not sorry

Gg

But ordinary comfrey spreads by runners more than seed so it might not be any better?

Thanks everyone, I certainly will take all the advise on board.

To answer the last part of your question. Comfrey has a deep root that prefers to be in soil. Containers are not the best situation for Comfrey. Infact I would advise against it. But then some one else may slap my wrists by contradicting me.