The first thing you need to know is what kind of players you'd like to buy and bring into your team. I prefer certain countries, mainly in Europe and I usually assign a few scouts to certain areas. Say, for example, you prefer players from the UK and Skandinavian players.
I'd recommend at least 3 scouts. One per region and a third one to scout your opponents and the occasional hand-picked player you'd like to get a scouting report on.
Since John Park is pretty good, I'd keep him arround and ditch Moss and Murphy. With Celtic, you should be able to get some better scouts. Since I'm not sure whether or not the stats for staff are the same for every new game. You could look for some scouts of your own with the Search Staff option and set the values for Judging Ability and Potential to 15.
There should be about 10 scouts with at least 15 for both Judging Potential and Ability willing to come to Celtic. A few names you could try are Gerry Collins, Ricky Sbragia (Scotland), Barry Whitbread, Glenn Cockerill (England), Glan Letheren (Wales), Eduardo Maciá (Spain) and Gianluca Nani (Italy). Celtic limits you to having 9 scouts, however, just try to get a few for the regions/countries you are interested in, one on the side for single targets and opponents and maybe one for a more worldwide scouting assignment.