Photocopying replacement pages

Mine is doubtless a recurrent question.  I feel it is worth asking
because practices change and interpretations of copyright law are
revealed such fun.  What parameters has anyone-who-is-willing-to-share
set on the number of pages that can be photocopied for replacement
purposes?  My concerns have much to do with potential damage to the
volume copied from in addition to tip-in versus rebinding considerations
regarding the volume the replacement pages are going into.  A brief
description of whatever logic you applied in setting limits for your
numbers of pages would be appreciated!  Thank you.  rs

                                  ***
                  Conservation DistList Instance 5:49
                 Distributed: Wednesday, April 15, 1992
                        Message Id: cdl-5-49-006
                                  ***