Let's assume that your pedal board is fully true bypass (so no Boss pedals etc).
If you run your passive instrument via a long cable into your pedalboard, then I'd put a buffer at the start. If you run 3-4 pedals, you're probably not going to need a buffer really unless you're using the aforementioned long instrument cable. If you're running a short cable from your instrument into 12 pedals, I'd pop a buffer in the middle or the end.
If you're running a long cable into a pedalboard of lots of pedals, I'd have one at the start and one at the end.
Bear in mind that some pedals (most notably Fuzz Face based fuzz pedals HATE a buffered signal fed to them (hence why they don't work well with active instruments).
Also have in mind that if you use any Boss pedals (there are others, DOD, some EHX, some boutique, B7K), basically buffered pedals, these work as your buffer, you don't need a little boutique buffer taking up extra space. A Boss tuner at the start (or wherever you need it as above) will usually do the trick.
Si