passing in table name as plsql parameter

You can use dynamic SQL:

create or replace function get_table_count (table_name IN varchar2)
  return number
is
  table_count number;
begin
  execute immediate 'select count(*) from ' || table_name into table_count;
  dbms_output.put_line(table_count);
  return table_count;
end;

There is also an indirect way to get number of rows (using system views):

create or replace function get_table_count (table_name IN varchar2)
  return number
is
  table_count number;
begin
  select num_rows
    into table_count
    from user_tables
   where table_name = table_name;

  return table_count;
end;

The second way works only if you had gathered statistics on table before invoking this function.