1 flow meter per controlled flow: one for each washer [if there is
no tempering valve you may want to monitor hot and cold separately],
one for each processor, one for the coffee maker -- just kidding.

They measure flow, not volume, as in gallons/minute, but I think that
is what you mean.  There are some that measure volume by integrating
flow: these are _not_ what you want.

Well, most folks try to make sure there is an adequate volume, but if you
insist ...

'Little blessings'?  That's a phrase I have never heard applied
to a flow meter.

I take it you are looking at meters of this type:

http://www.omega.com/ppt/pptsc.asp?ref=FL2000&Nav=greb02

Sometimes called a 'Dwyer' meter, after the name of the most common maker,
sort of like Kleenex.

The upside is

o the right amount of water is flowing to wash prints etc.
o water isn't wasted

The downside is

o the meter may cost more than the water saved,
  though if it keeps prints from staining it's cheap

I occasionally calibrate my 'flow sense' by filling a litter graduate
while timing how long it takes to fill.  I can usually set flow to
within 20% now that I know what 1 gal/min looks like when it comes out
from the tap.

The problem I have is the valves creep and water flow changes
a lot over time and so I have to recheck every few minutes until things
become stable.

If your washers and processors are more-or-less permanently plumbed
into the water line then I think flow meters are a good idea.

If you use a rubber fitting or quick connect to hook to the faucet and
your valves and water supply are stable then checking the flow visually
every now-and-then is probably good enough.

I would check ebay for flow meters.  Don't pay more than $5 for a used
one.  Make sure it is not a 'gas flow meter': you want one for GPH, not
SCFH or LPM. If you know someone who works in chemical, food, power etc. plant
they may be able to get you some from the scrap heap [no five finger
discounts: most firms have a heap of old junque].

The nice meters have a little control valve on them that makes adjusting
the flow very easy.  Plumbing taps/valves are not made for fine
flow control.

I use really big ones in industrial settings, and really tiny ones
in laboratory instrumentation.  Not the same sort of visual ball-in-a-tube
meters, though.

I have a flow meter/temperature mixing/filter panel in both my darkrooms. In
the RA-4 minilab room, the minilab calls for a flow rate of 2GPM. The
plumbing allows a max of 6GPM, which I use for filling the tanks, then I
bring it down to 2GPM.
In the manual/film darkroom, the flowmeter lets me know if the water flow is
sufficient for the wash rate I want/need. If I want 6 water changes in a 3
minute time period and I'm using a one gallon tank, then I need 2GPM.
I would think that you need a flowmeter for each device that requires a
measured flow. If you were running several devices, I suppose it would be
possible with the proper plumbing to set up each device with a temperary
flowmeter, then remove it.
Ken Hart