Generally you can fit one standard valve in the 6" circular valve boxes. The jumbo boxes I believe you are talking about are the 12" rectangular boxes, and I wouldn't put more than 4 valves inside that. The spacing is really your discretion, but I would leave enough room so that maintenance on the valves isn't a pain in the rear. I would also suggest the threaded valves over the slip on, strictly because if a valve goes bad and you need to service or replace it, you're not having to cut out the slip on valve in an already cramped valve box. With the threaded you can just unscrew it.
Tony Posey
Ridge Run Landscapes