Sorry, I forgot to mention about it, it's SLC 5/05 with 1-slot addressing on the existing one, this one will be added to the system and I have a 10 slot chassis available.
If it's physically close enough, you can just extend the local rack with a cable that will make your second rack just look like an extension of the first one...no scanner card, adapter card and update delays. This is good up to 30 slots total.
If all you have is one 13 slot rack (you didn't say what you have already) and no RIO racks, then you'll need the 10 slot chassis, a 1747-ASB and a 1747-SN. The SN module will need to be located in the main chassis. The ASB goes in the new rack where the processor would normally sit.
If you already have some RIO on an existing SN module, let us know what's there to make sure you don't exceed the capacity.
EDIT: Selection Guide with info about the number of I/O the SLC can control.