can players (non-faction) own them. I know only curtain factions can use assigned data cards. Just wanted to know; say any type of ship DC can be used at any Shipyard or item/factory...ect.
here is the thing- I wanted to buy a hidden base DC but was told you needed to be in only that faction to build it. True or no?

You can't buy datacards (yet, combat may change everything). You can often buy uses of datacards from the faction, depending on how they do business. You usually pay that faction to build it for you.

Station datacards can indeed only be used by those in specific factions.

I was just wondering...
If I would like to build something, like... a droid - all I need is RMs, Factory and to have assigned datacard, right? Or is there something else too?

RMs, a factory, the assigned datacard, membership in a droid production faction, privileges in said faction, money to produce, worker NPCs.

oh... I have to be in faction too... damn ~_~

how many datacards are there

Every entity that can be produced has a datacard. If you want the exact number, add up all of the produceable entities on the rules pages; they should include ships, weapons, droids, items, and vehicles.