reftex-multiref-punctuation

reftex-multiref-punctuation is a variable defined in `reftex-vars.el'.
Its value is

((44 . ", ") (45 . "--") (43 . " and "))

Documentation:
Punctuation strings for multiple references.
When marking is used in the selection buffer to select several references,
this variable associates the 3 marking characters `,-+' with prefix strings
to be inserted into the buffer before the corresponding \ref macro.
This is used to string together whole reference sets, like
`eqs. 1,2,3-5,6 and 7' in a single call to `reftex-reference'. See manual.

You can customize this variable.