It can be a challenge having a 1-2 player group which I’m sure each one of us has had at some point, unless your Dungeon Master Charisma score is extremely high.
There are many reason why you might have only 1 or 2 players. The reasons may vary from you just don’t have enough players or you’re leveling up a new player separately so they can join the main group. As a DM, setting the adventure stage with 1-2 players is not as difficult as you might think and sometimes having a small group can be a ton of fun. Just think of the benefits; like not having to scream at the top of lungs that there are spider webs covering the ceiling to the Wizard who is currently occupied with his dice and pencil at the far end of the table. You will become the story master for a single player who most likely will need help staying alive. Yes, staying alive depending on the class of the character will be a huge challenge. Strength in numbers has never rung so true, and not everyone wants to play the Bard who has access to multiple skills such as healing. So as a DM, the life of your adventurer is completely in your hands.
Running a group of 1-2 players can be extremely fun, and having less players to manage has its own rewards. The game will run a bit faster but you’ll have more interaction. The largest risk with a small group is death. If you are a fan of rolling dice in front of your players for combat you will soon find yourself spending a lot of time rolling up new characters. It doesn’t take much for a Goblin with a sneak attack to remove your player from the game. My main rule for any D&D game: I never roll dice in front of the players. Why? As a DM you are creating heroes and stories for them to experience and enjoy. One roll can turn your player’s mighty Barbarian into the next zombie they encounter. Killing off characters is bad for business. Your reputation as a death dealing tyrant will endlessly haunt you.
Some simple rules for the 1-2 player group. Give them items to help them survive. A couple of potions of healing from a benevolent quest giver can go a long way. Something as simple as a +1 weapon at 2nd level can improve their chances against the many foes you will have lined up. Use your imagination and set out adventures specific to the player(s). You can also add in your favorite NPC to boost the party. There is nothing wrong with playing the Wizard’s side kick, it can help drive the adventure course. If you have purchased a premade adventure, learn to adjust the number of nasties and abilities of the villains to match the number of players. It’s not fun for the single player who ends up getting charmed and it often results in Game Over.
Now it’s time to address the leveling of a new player to the group. I have done this very regularly for a number of reasons but I don’t do it all the time, it’s all based on the player. First and most important, I’ve never liked new players starting a few levels higher. They may be starting a new class that they have never played before, and leveling them from 1 onwards gives them a chance to learn the class and develop a character based on the decisions they make. 5e does a terrific job at skill/ability progression and the first 3 levels of each class are very important to the future of the character.
When you start a player off at a higher level, they often don’t understand all the wonderful things they can do and they have not developed a play style suitable to that character. By starting a 1st level character off with a party at a higher level you run the risk of character death. We all know of that young green dragon that uses his breath as a weapon on the party, and we know as a DM that the half damage save won’t do anything for the level 1 Wizard with 8 hit points. This results in a Wizard who is now dead in a cloud of noxious gas, and a player who is facing having to roll up a new character or reconsidering playing the game. This is why I’ve always run a separate session for the new player to level them up. By giving that character a better background to the adventure, you now have a player who has a better understanding of their character and class. The bonus to you as a DM? You almost never have to suffer figuring out how to save the character from certain death and you make it believable.
There are a few premade adventure for single players out if you care to search for them. Use them or take them as examples and create your own. Either way, go enjoy and build your stories.