I would be very hesitant to host multiple new dogs together for an extended period of time. I'm experienced at reading body language and feel comfortable introducing dogs and managing behavioral issues; however, it's not worth the money if you have to spend days or weeks managing problems and preventing/resolving conflict, even if your house is equipped to handle a quarantine situation. And perhaps even more importantly, even if you feel able to handle a worst case scenario, remember it's not ideal for the dogs. As a sitter, I don't want to watch dogs just because I am capable of doing so. I want to watch dogs who will thrive with me, more than they might with other sitters. I'm happy to work with difficult dogs who might be uncomfortable with any sitting situation, but if there's something in my house that makes them uncomfortable which wouldn't be an issue at another sitter's, I'd rather that dog go where they aren't uncomfortable and can have a good time.
Last year I hosted two dogs over Christmas and New Year from different households, but I was only comfortable making that arrangement because the M&G for the second dog happened to coincide with a stay with the first dog, so they were able to meet well ahead of time. Thankfully, they immediately loved each other and I had no concerns about having such long overlapping stays. For short overnight stays or daycare, I'm fairly comfortable mixing familiar guests with new dogs, though I always consider the personalities of those involved and I've declined new dogs when it didn't sound like their personality would mesh well with the existing group.
I also echo what Frances said: if you aren't very experienced or aren't comfortable handling multiple dogs, or aren't set up to keep dogs separated safely, do not accept overlapping stays, especially not for long periods of time. When you have a few trusted regular clients, begin allowing small overlaps between their stays as you can, until you become more confident with introductions.
What you might consider, since your first client will be booking ahead of time, is asking your first client if they might be willing to meet again later if/when you have overlapping requests so their dog can meet the potential second guest. It may be harder to arrange, but it could potentially allow you to host multiple dogs and alleviate many concerns about compatibility. If you do take multiple clients, still be sure you have the ability to safely separate all dogs in your care should it be necessary during the stay.