How to fix this? when input none to a sqlplus substitution parameter

Question

a small program:
ACCEPT p_input PROMPT 'Please enter the number of top money makers: '
DECLARE
v_count NUMBER := &p_input;
BEGIN
DBMS_OUTPUT.PUT_LINE('This is only a short program for asking the question. ');
END;
-- To run the sql:
SQL> start quest
Please enter the number of top money makers:
-- When just hit Enter keyboard after the prompt, the following error message displays:
v_count NUMBER := ;
ERROR at line 3:
ORA-06550: line 3, column 21:
PLS-00103: Encountered the symbol ";" when expecting one of the following:
( - + case mod new not null <an identifier>
<a double-quoted delimited-identifier> <a bind variable> avg
count current exists max min prior sql stddev sum variance
execute forall merge time timestamp interval date
<a string literal with character set specification>
<a number> <a single-quoted SQL string> pipe
<an alternatively-quoted string literal with character set specification>
<an alternatively-quoted S

Answer 1

What did you want ? no value or 'null' value ?
If you want to manage the two possibilities, try something else like below :
SQL> ACCEPT p_input PROMPT 'Please enter the number of top money makers: '
Please enter the number of top money makers:
SQL> DECLARE
  2     v_count NUMBER := to_number(case when '&p_input'='null' then 0 else nvl('&p_input',0) end);
  3     BEGIN DBMS_OUTPUT.PUT_LINE('This is only a short program for asking the question. ');
  4  END;
  5  /
old   2:    v_count NUMBER := to_number(case when '&p_input'='null' then 0 else nvl('&p_input',0) en
new   2:    v_count NUMBER := to_number(case when ''='null' then 0 else nvl('',0) end);
PL/SQL procedure successfully completed.
SQL> Nicolas.