My understanding is the Speaker has to have 50% or more votes. They have to agree one or they cant officially function so if there is no agreement (or +50) after a first vote they go into multiple ballots until a candidate gets over the line. No other business can progress. It can go on for months like this.
Congress due to start on the 3rd Jan and they would have first vote on speaker that day. But they have a looming spending bill fairly soon so they have to have a speaker to avoid a shut down.
Its highly unlikely that the Republicans will end up seeking support of Dems to get a speaker elected. The Dems are not going to help McCarthy. Its hard to see how anyone else could come forward and overtake him, even with some Dem support you need 200+ GOP votes. Could a handful of GOP help a dem candidate, very unlikely, they would effectively become independents. A third possibility is a non elected speaker, a random could come forward and take whatever votes - again highly unlikely as no candidate.
The likely outcome is a GOP shit show and a much damaged McCarthy getting the job. The question is how much damage to Gatez and co want to cause?
But the Reps and Dems each have a caucus to choose their candidate (and the Dems at least are supposedly bound to support their candidate). If the Reps all don't vote, then there would not be a quorum. Then what?