- Is whole Stockholm subway in one zone (A zone)?
- How much does cost one ride for Stockholm metro?
- Where I can buy tickets?
- No, there are multiple zones, mostly you have to cross from one zone to another, hence pay more. Here's the metro zone map:
They sell 16-ticket slips for 200 SEK, you need two units (tickets) if you're traveling within a single zone, three units when in two zones (e.g. from A to B) and four if you're traveling between three zones.
The easiest way would be getting the tickets at stations, there are vending machines and also sometimes cashiers selling those tickets.
All of the subway stations are within zone A, but some buses and trains reach outer zones.
A single ride is 45 SEK, but if you have a prepaid travel card (costs 20 SEK) it goes down to 32 SEK. Paper coupon strips are no longer accepted.
You can buy a card at the Central station SL center among other places, and fill it with cash or tickets at the vending machines in every station. Check http://sl.se/en/fares--tickets/ for details.
The simplest way is to get a SL access card from any Pressbyran store. There is a small admin fee, but it means you can just touch the card at the ticket barriers for the Tunnelbana. With a SL access card, the single fare for a journey on the Tunnelbana is (in January 2019) 32 krona. You can also use the SL access card on the Pendeltag (commuter rail), trams, commuter ferries in the city centre (but not Waxholmsbolaget ferries, unless you have certain types of pre-loaded travelcard), and buses.
Using the SL access card, you can also combine modes of transport under one single fare, provided that the last leg of the journey is commenced within 75 minutes of the first leg of the journey having been commenced. You can see what that single fare would be from the journey planner at (in English): https://sl.se/en/