as long as you set the system up with proper safety equipment, like a window switch, cutoff switch, etc and ensure you have adequate fuel to add when you spray you can run a fairly large amount of boost.
there are plenty of places that sell whole kits with all safety equipment.
for some nitrous info...
http://www.automedia.com/Nitrous_Oxide/pht20020301no/1
as far as the amount to spray safely... id start at around 100 or so, then do maybe a max of a 200 direct port shot with a modified fuel system just to be safe. plus running a step colder on the spark plugs as well as gapping them about .005 tighter.