[OP ...] carbon has 4 valence electrons so... it needs 4 more to fulfill the octet rule right?
Yes, that is correct.
If a double bond yields 4 electrons, does it mean that the carbon atom can make only one double bond? Or does it mean it can create up to TWO double bonds (because they would yield 8 electrons which fulfill the octet rule)?
The easiest way to do this in your head is to assume that for each bond, one electron "comes" from the carbon atom of interest, and the second "comes" from the bonded atom. So carbon often makes four bonds (which could be two double bonds, as in carbondioxide, in allenes or cumulenes). There are less common cases where carbon has a positive or negative formal charge but it is useful to first become familiar with the common cases (where carbon forms four bonds) before you move on to those.
What about Calcium? I ask because Calcium doesn't follow the Octet rule: it only wants to have 4 electrons, so...
Calcium typically does not form covalent electrons. In many compounds, it occurs as ion with two positive charges. Instead of completing the octet, it forms ions with a completely empty outer shell (so the fourth shell is empty, and the third one is full, with an octet).