So I have a number of Brew sessions in planning for the summer that we require need to brew about twice as much as I normally do. I normally do about 5 to 6 gallon batchs, however I have a couple of batches that will be between 10 and 13 gallons. I currently have a mash tun that holds 10 gallons of water and comfortably does a 6-7gallon batch. Should I buy a second mash tun so I can do a large amount at once or just do two separate mashes and use one mash tun. I do have a 15 gallon brew pot so that should not be a concern, and I also have a 16gal fermenter.
I like to be able to do full mashes in one vessel, but I know that some folks will do a concentrated mash and simply add water to the wort for the boil. I also know that some RIMS arrangements use 2 vessels for the mash - one with the grain and another that is being heated and returned to the one with the grain by pump. You balance the flow and return, such that about half of the wort is in each vessel (and return lines) at each point in time, until running off into the boil vessel. (I think the Blichmann approach is the version I have seen emulated - Blichmann calls it the Brew Easy, I think and the RIMS approach in that setting as K RIMS (Kettle RIMS).
I use a 10-gallon Igloo mash tun. I can do 10-gallon batches of beers of 1.055 or less. When going for something bigger, I just add a pound or two of extract to the kettle.