I just really miss when there was more than 2 models. When they had 2 episodes so far this year that had all 6 models, I was downright spoiled. I get budget constraints, but if that's the case, why even have a cast of 6 models when we may go weeks without seeing one of them?
They have so many models so they can rotate them in/out when needed. They know that the models had other opportunities before they became models on the show and that they'll want to have access to other opportunities outside of the show both personal and professional. For example, James isn't taping right now because he's gone back to Australia to film a movie, and it isn't the only time that he's taken time off to film something else.
Rachel, Amber, and Manuela have all been pregnant and taken leave from the show. Manuela is on maternity leave right now, and we're down to about one or two episodes left before we won't see her on the show for a while. So, while one model takes leave, there's other models that are available to take their place.
Rachel and Devin both live outside of Los Angeles and fly in to film the show. Rachel's in Louisiana, Devin in NYC (formerly Baltimore). So, sometimes they're not always available to film at a moment's notice. In fact, being available in a pinch is why Alexis was hired.
All this said, I wish they would do more three model shows. The four model show days of the 90s always seemed a little excessive (though make no mistake, I loved Kathleen), so I don't think they need to go back to that, but three model shows definitely feel nicer and fuller.