How many 16 gram CO2 cylinders for a 17" tire?

Regarding those CO2 tire inflator kits, anyone know how many cylinders it
takes to get a rear 180x70x17 tire back up to 40 psi?

Dealer says a lot, maybe 7-8 cylinders.

Dealer might be right. I had to use the one that came with a BMW and three
didn't even make a dent in it. Of course, some leaked out of both the the
stringy stuff they call a plug and some more from the funky adapter for the
C02 chuck. Ended up needing to use a can of Inflate-a-Flat in a larger
spray-type can and tire still wasn't fully up and wobbly.

I believe after the ordeal those C02 things were designed for bicycle tires
and not motorcycle tires. They say that one 16 gramer can inflate a
'Mountian Bike (i.e. bicycle) Tire" to about 40psi. That's going to be a
lot of motorcycle tires.

Get a compressor like maybe the "Slime" one. It's volume would amount to a
dozen of those cylinders and you'd still have fudging room should it still
leak. It doesn't cost any more than a dozen of those C02 things.

Anyone need to use one?

I don't think you need 7-8 cylinders, but a pump
would be less bulky if you did.

I normally use a $10 Walmart tire inflator with
the plastic housing and fan removed. At a rally
last weekend, I saw the same $10 Walmart pump
reworked a little more elegantly than I'd bothered
with selling for ~ $45.

Wire an ordinary lighter socket into the electrics
to provide an outlet to plug the pump into. Also
very useful for quickly inflating your air mattress
at night when you pitch camp.

Not yet. I think BMW packs two or maybe three in their puncture kits
(I'd have to check), and so I'd assumer they know the reality, and pack
that many.