I covered most of this in card template, so what you mainly need is over there, these are rules the dm has to enforce with the players. So when creating a new character a new player is only allowed to start off with one. They must then complete at least one side quest with said characteror one main quest if no side quests are available. What that character is allowed to be, do, and start off with must first be decided by the player and then checked by the dm to see if its ok. Pets are allowed but only w/i reason, the same goes for summons. Once again most of this is covered in the card template. There is a rule that a character can be more than one person, such as if the character is a duo, or if one character has a small guide with them and or a talking item, or if that character is an almagamation of souls. Normally the limit is up to 3 people out at once representing one character. Now if players are making a solid character or an almagmation of people as a character than if they met the previous prerequisite of playing one whole quest then they are allowed to make more, but can only swap out characters at the beginning and ending of quests, as to who the swap happens that is mainly up to the dm. One last thing I want to note is that normally a player can have up to 4 characters out at once, if they created more than that they are allowed to say that the ones they aren't using are part of a group thats working together and/or are off doing something else while the main story is taking place. There are different limitations however depending on how many players are joining you for a session, tabletop simulators limit is 10 so i will make a chart below explaining the player to character ratio. If you have people dropping in and out just use the ratio you started with at the beginning of a quest and then change it when the quest ends. You count as a player if you have your own characters taking part. The reaons being there isn't always enough room for everyone and its hard to keep track of every party character especially when there are npcs tagging along.

Number of players Min to max number of characters allowed for each player to be out for that session.
1-2 1-6
3-5 1-4
6-8 1-2
1-10(whether there is/isn't room for dm characters) 1 only