From what I understand about California recalls, and Bob can help me here, everyone running for the office is on one ballot. If the majority supports the current governor, he stays. If they don't, then there is a second question on the same ballot to select one of the various candidates. There's no runoff and whoever gets a plurality will win.
It would behoove the Democrats to find one strong candidate to put on the ballot so if Newsom is recalled, that candidate would probably outpoll a widely split group of Republicans, independents, and fringe candidates.
Here in Georgia, when we had the special election for Senate, there was concern that a group of Democrats would split the Democratic vote and allow the two main Republicans, Loeffler and Collins, to make the runoff. However, they quickly coalesced support around Raphael Warnock, who got an easy plurality of the votes against the nine Republicans (and various other fringe candidates) who were running.