The easy way would be having every session be a one-shot*. No issues with XP and loot-sharing, no issues with having to deal with PC left "mindless" because of absent players, no issues with several people having to catch up on the story every now and then because they missed the previous adventure, and many more problems easily resolved.
However, this may get boring at some point. If you want to plan a great adventure or even a campaign instead of a dungeon, only organising one-shots surely wouldn't help. The main problem would be the inconsistency of the players in the "main" party. There are a few ways to solve this:
- Have every session end in a busy place. This is to make sure it's logical when the "main" party gets switched in and out, as long as there is at least one player/PC left from the previous adventure. For example, you end in the main city and the rogue of the party couldn't keep his greedy fingers in his pockets and is now stuck in jail, or the wizard wants to read every single book in the library to make sure he's not missing out on any valuable information.
- Have your players be part of an organisation. Your party-members are all part of a great Guild of Adventurers, or are all mercenaries. The down-side to this method is that if your planned adventure takes longer than one session, you can't switch between the players in the party without a very good reason. More people could of course join the group and be promised a part of the loot for example, but you can't suddenly have people leave in the middle of the adventure, unless there is a very good reason for it. For example, the character got kidnapped, and is now held hostage by the main baddy.
- Make sure everyone has a back-up character. If half of a group of mercenaries disappear without a legitimate reason, it may be a good idea to simply pause this campaign and have the players join another. Having a back-up character sheet at hand will make this a whole lot easier.
There have also been asked many questions on this site about how to deal with one specific player often being absent. I'm afraid I can't link them right now, but looking those up might prove to be a big help to you as well. For example, How to RP the character of an absent player? (1 2 3)
Inconsistencies between party-members
Now, another problem you may encounter is differences of loot and XP between players. I can only advise you to use the system of group-XP and group-loot. This means that you don't track XP and loot individually, rather as a group. When a new PC (re)joins the party, make sure it has the same level and amount of loot as the rest of the party.
I won't say this is a bad solution. It is, however, a bad idea unless you have someone else DM these campaigns. I advise against you running several campaigns at once, especially when you're new to this game. If you think you need several campaigns, look for someone willing to DM the others.**
*One-shots are simply dungeons or adventures that take no longer than one session. This means the players can play new characters every session if they want, and consistency between sessions is not necessary.
**Do note that there is a significant difference between adventures and campaigns. One DM can prepare several adventures in one campaign, but preparing several campaign is almost too much work.