How many Washers to put in on top of each other?

About twenty five years ago we had the diaphragm toilet washer replaced on our Twford toilet. [ The type with the little disc at the top in the middle of the tank that you pull vertically upwards to flush it].
The person who did it for us before is no longer available, so i have (as a complete *novice*) to do it myself. I remember dozens of bits all over the Lavatory floor, so I'm not looking forward to this mammoth task.
I also remember lashings of 'Plumbers Mate', so I'll get some of that. Also just now on searching for a washer, there is talk of a foam doughnut washer as well, so i guess i should get one of those?
I remember he actually put two washers in, one laid on top of the other. He said it would last longer, as one washer might give out, but you would still have the other. Since i don't want to ever do this again in the few years i have left, how many washers could I lay in on top of each other before it becomes counter-productive?
Any tips or advice up front, i would be very grateful for. Thanks.
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email.me:

Your description may be a bit too vague - a photo would help.
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On 24/09/2015 17:03, john west wrote:

Like this?; (Amazon.com product link shortened) Colour may vary.

Plumbers Mait shouldn't necessary.
You need one of these;
http://www.toolstation.com/shop/Plumbing/d20/Toilet+Fittings/sd2835/Close+Coupling+Kit/p85615

If 2 washers lasted 25 years, it depends how old you are :-)
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