If your intent is to fill out a City's cabin, even just a single 8" would do the job. You're not into gapang naman (and even if you are, it'll still be enough for the average listener). But to be safe, I'll suggest a single 10".
While two 8" would be theoretically be better, a single 10" would be better for your wallet
Re: Active Subwoofer
Actually if you want the best active for the money..
1. Know your budget
2. Check out the "products"
3. Pick the "product" with the biggest cone size/space taken.
Speaker performance is highly dependent on enclosure size. That's why (in general) the bigger it is, the better it is. Now you just need to find a product that fits into your space requirements.