Do you have Global Forever stamps?

I don't think one will be enough. USPS have quite a few rules on sizes and what can and can't be sent at base value. If it's a board envelope or you include card then that would likely tip it over on weight.

Check the price calculator at http://www.usps.com. Stamp values can be found there too, or just use Google :)

I am looking to send out some 8x10's in A4 envelopes from the UK to USA.

I have got some forever stamps to put on the return envelopes.

Will one forever stamp be enough on an A4 envelope or should I go with two or more?

Thanks.