This is a tricky question, due to procedural generation. No Man’s Sky has the potential to have 18 quintillion planets, specifically: 18,446,744,073,709,551,616, which is as far as 64-bit computing can take us. However, these planets technically do not exist until they are first discovered by a player, which causes the system to generate their traits and populate them with fauna, flora, and points of interest.

In short: NMS has the seed potential for generating 18 quintillion different planets, but only a small fraction of them has been actually generated.

Despite the game’s tremendous galaxy-generating potential, according to its Steam version system requirements, it requires only about 15 GB of available space. The size of an actual installation might be larger due to numerous content and technical updates.

Yes it does, told over the course of various transmissions and conversations with NPCs you are told a fellow traveller and the reality of the cosmos you find yourself in. Going with the plot also acts as an extended tutorial and will bring a decent number of resources your way. However, you are also free to ignore it entirely for any amount of time.

You can have twelve active ships and one freighter.

The game saves automatically quite often, most notably when you exit your ship or interact with a beacon frequently found near various structures on any given planet. Additionally, when you unlock the blueprint for the Save Beacon, you can make saves even when you’re ranging far from your ship or any structure. As a bonus, the save beacon will also appear on your radar, which makes it useful for marking otherwise easily lost locations.

At the core of No Man’s Sky is exploration of the galaxy, one planet and star system at a time. In the process, you’ll also:

• mine resources both planetside and in outer space;
• trade with NPCs, learn alien languages, and build reputation with factions;
• build a base of operations, or several;
• create a small fleet of personal spaceships by restoring them from wreckage, buying them from NPCs, or constructing them piece by piece;
• catalogue minerals, fauna, and flora on discovered planets, including giving them names if you’re the first one to encounter them;
• discover technologies to make your ships, vehicles, spacesuit, and omni-tool more powerful and versatile;
• manage settlements;
• …and more.

Yes, you can play offline. Some functionalities might not work fully if you do, but the bulk of the game will work just fine.

If your intent is simply to avoid meeting other players, you can disable multiplayer in the “Network” section of game settings.

Yes it does! No Man’s Sky offers crossplay between PC, Xbox, and PlayStation.

You need their No Man’s Sky friend code, which you (and they) can find by going to the main menu, selecting the “Join Friends” option, and clicking the “Show my No Man’s Sky Friend Code” button. When you have your friend’s code, or they have yours, they can enter it after clicking the “Add No Man’s Sky Friend” and when both of you are online, your sessions will appear on the list in the “Join Friends” menu.

It isn’t, it has a hard technological limit to how many planets, systems, and galaxies there are. However, that number is so high that from an individual experience the difference is academic: nobody can feasibly experience that limit.