Why are there 40 commandments in 10 commandments?

Question

When do one commandments stop and the next one start? Who's counting? Who got the number 10?

1. I am Hashem
2. your God
3. You shalt have no other gods before me
4. You shalt not make any graven image (So no 3D printing?)
5. You shall not bow down to them
6. or serve (them)
7. You shalt not take the name of the Lord thy God in vain
8. Remember the sabbath day
9. to keep it holy
10. Six days you shall labor
11. And do all your work
12. but the seventh day is a Sabbath to the LORD your God.
13. On it you shall not do any work, you
14. or your son
15. or your daughter
16. or your male servant
17. or your female servant,
18. or your ox
19. or your donkey
20. or any of your livestock,
21. or the sojourner (non jews?) who is within your gates,
22. that your male servant
23. and your female servant may rest as well as you.
24. You shall remember that you were a slave in the land of Egypt,
25. and the LORD your God brought you out from there
26. with a mighty hand
27. and an outstretched arm.
28. Honor thy father
29. and thy mother (2 commandments)
30. Thou shalt not kill (actually murder/certain types of killing presumably against other humans only. Many killing, is perfectly fine and encouraged. Just read http://biblehub.com/1_samuel/15-3.htm)
31. Thou shalt not commit adultery (Most disagree on what it really means)
32. Thou shalt not steal
33. Thou shalt not bear false witness against thy neighbor (literally comrades)
34. You shall not covet your neighbor's house
35. you shall not covet your neighbor's wife,
36. or his male servant,
37. or his female servant,
38. or his ox,
39. or his donkey,
40. or anything that is your neighbor's.

Some may think my division to 40 is excessive. Still if I shorten it, it'll still be way more than 10. Most denominations divide the commandments differently anyway, suggesting that the division is not clear cut.

Answer 1

That which you call the 10 commandments are in Hebrew referred to as the Aseres HaDibros, which means the 10 utterances or 10 statements. There is no implication that there are 10 commandments therein. I think your 40 is a bit excessive (as you conceded), but the classic counts list either 13 (Behag) or 14 (Rambam) commandments in these 10 statements.

As an example that they are not one commandment each, it is actually the opinion of several major authorities (Behag, Ramban Hasaga 1 to Sefer HaMitzvos) that the first statement is not a commandment at all.

Answer 2

"The Ten Commandments" or Decalogue is not a Biblical concept - and you won't find a reference to it in the Tanach.

So your question makes an assumption - based on a popular mistranslation!

The closest you will get is a Pasuk in Devarim 10:4

וַיִּכְתֹּב עַל-הַלֻּחֹת כַּמִּכְתָּב הָרִאשׁוֹן, אֵת עֲשֶׂרֶת הַדְּבָרִים, אֲשֶׁר דִּבֶּר ה' אֲלֵיכֶם בָּהָר מִתּוֹךְ הָאֵשׁ, בְּיוֹם הַקָּהָל; וַיִּתְּנֵם ה', אֵלָי

Which roughly translates [with my comments] to: "And He [Hashem] wrote on the [second] tablets as was written in the first, the 10 items that Gcd spoke to you on the mountain [of Sinai] from within the fire on the day of the gathering and Hashem gave them [the tablets] to me."

How to get to 10 is a matter of dispute - and you can see 7 various versions from various schools of thought on Wikipedia.

See the answer from YEZ above for some Jewish opinions.

Regarding your query as to who decides what is a Mitzva - that has rules, as discussed in the Talmud. A popular set is Rabbi Yishmael's 13 principles we say in Shacharit every morning. Or see Rules by which early Jewish law was derived.