I'm going to assume you mean combinations - the unique set of these letters in any order with no sequence repeated. With these letters, there are 60 possible combinations. To see the maths behind this, try typing "permutations of {c,c,c,p,p,k}" into wolfram alpha.
more than 1
The 4 letters can be arranged in 24 different sequences.
There are infinitely many possible number sequences, and infinitely many numbers which can appear in those sequences. Any and every number can appear in a number sequence.
You have 2 choices for 20 times, so the number of sequences is 220 or 1048576.
If you assume 26 different letters, there are 26 to the power 5 possible variations of 5 letters.
If the numbers and letters can be repeated then there are 45,697,600 possible outcomes. If the letters and numbers can not be repeated there are 32,292,000 possible outcomes.
24
Because there are 7 letters that don't repeat, 7X6X5X4=840 combinations. To give you an idea of how to solve the problem, if there had been 9 letters and you wanted to make combinations of 3 letters, you would multiply: 9X8X7=504
There are 6 sequences in the metabolic pathway of Glycolysis
12 memory sequences
There would be a possibility of about 16 sequences.
Assuming the the last part of the question is that the sequence may not END in 000, there are 2997 sequences.