SELECT COUNT("Rider") FROM table_74374 WHERE "Tues 31 May" = '18'' 55.91 124.236mph'
What is the number of riders that had a Tues 31 May time of 18' 55.91 124.236mph?
CREATE TABLE table_74374 (
    "Rank" real,
    "Rider" text,
    "Mon 30 May" text,
    "Tues 31 May" text,
    "Wed 1 June" text,
    "Thurs 2 June" text,
    "Fri 3 June" text
)