How many entries in PDB have SS bonds?
The easiest way is to check annotations in mmCIF or PDB files.
Looking at my page with statistics of mmCIF tags, last updated in March:
_struct_conn_type.id        59137×metalc 42706×covale 36195×disulf 10725×hydrog
_struct_conn.conn_type_id   5350402×hydrog 1733277×metalc 559092×covale 226472×disulf

Which means 226472 disulfide bonds in 36195 PDB entries at the time when the PDB had 161470 entries.