You should also read the General Guinness Record Guidelines . This is a solo event, for stamp licking in a time of 5 minutes. The stamps must be in the top right hand corner of the envelopes so that they could be cancelled by the normal machine impression.
Torn stamps are acceptable provided more than half is affixed in the appropriate position.
Stamps can be torn from sheets before the event so you are actually holding individual stamps or stripes of 10 etc. If inadvertently two stamps are put on one envelope, the attempt can still continue, but only one of the two stamps would count. The person attempting the record also has to separate all of the envelopes within the 5 minutes. Any envelopes which are not separated will not count. Previously, there was a different set of rules.