I have 4 not including a multi-plier.
So my questions will be:
"Will this be the only knife you have on your person ?" ,
"What are you using the knife for?",
and
"How much room do you have on your carrier for it?"
Weight considerations are always something to look into, as in my own case I have level 3 vest of kevlar with front, rear, and side plates and my carry weight with nothing at all in my vest is about 22 and a 1/2 pounds. I'm a big guy so this isn't too much of a problem but I'm not going to be running any 100 yd dashes very fast fully loaded.
If you are in public your knife should not be shown anywhere on your vest specially if you have contact or close proximity to the public. Actually I should say discreetly carried. You won't see a knife openly carried or recognizable on an LEOs vest. So I would suggest something that could be concealed that is of sufficient size to do what you needed to do. If you're looking for something with 4" or less there are numerous good choices out there. Tuck it in behind your magazines or into the side of your vest or underneath to where you can reach and get it if you're in a close clinch with somebody where they can't get ahold of it.
And yes, SOG seal pup is a good choice and it falls into that category and it's also got a proven record and not very expensive.