I reckon it'll be about 20mm....
My advice with spacers is just to make sure you have pristine spigots..
and that the wheel bolts are long enough.
Don't fit spacers that don't have a machined spigot (hub flange) to support the weight of the wheel.
A lot of cheap spacers are just a machined disc of alloy with holes drilled into it, which is great but it means the weight of the car becomes supported entirely by the wheel bolts, rather than the hub.
I would also only fit spacers that are bolted to the hub face but whose wheel bolt holes line up with the bolt holes in the hub. Apart from you get a touch more support, the spacer won't fall off every time you remove the wheel (and it's easier to align the spacer when you want to re-fit the wheel).
The new bolts should be the spacer's thickness longer than your existing bolts. Don't use your current bolts unless the spacer is 3mm or less thick.
The biggest issue with fouling is likely to be on the front, under load at full lock. The further out the wheel is, the bigger arc it makes as you turn the steering, so have a look at how close the outer shoulder of the tyre gets to the wheel arch liner as you (or someone else) turns the steering to full lock. If possible, stick your 20mm spacer on the front to test it while your chubby mate is sitting on top of the front slam panel, so you can see how close it gets.
The rear wheel has plenty of room around it and you'll see whether it sticks out close to or beyond the wheel arch lip. If it does, it will foul, so don't do it.
My preference, over wheel spacers would be to just fit wheels with a bigger offset (smaller ET number). You'll get the same effect but without all the additional ironmongery... although the fouling issue still needs to be checked.