Sussex should do 12. I had a marran sit on 18 under a hedge and hatch 15 once (she went missing till we heard the newly hatched chicks!). I'd prob just go for a doz under a nice big Orpington as well.
Broody will stil tight for 4-5 weeks, easily. so long as she knows to come off and feed well. (I've had some that would sit for days, either foul the nest or half starve themselves for days - these were not allowed to sit till the following year.)
I didn't understand the second bit of your question, sorry! "...currently sat eggs at work..."
I much prefer hen raised chicks. So I like to pop some eggs under the hen, and some in incubator on the same day and then consolidate all the chicks under the hen if possible! I had a Sussex brood 20odd the other year, think it was 26, I wrote about it on here. So if you have eggs coming that you need to incubate, I'd remove any she's been sitting on for a week and get her on the good ones, and any extras can go in the incubator on the same day.
Sudanpan: I would break her broodiness ASAP. Some hens will sit for months and lose a lot of condition in this time. If you don't want chicks I'm not sure there's much benifit to leaving them broody at all?
Put her in a wire cage, suspended off the ground with food and water. you need to keep the rain off her, or put the cage in a shed or garage for a couple of days. Listen for her voice to go back to normal.