Possibly because Odo is a fully developed Changeling (with lots of experience to enrich the Great Link).
The Infants they sent out might actually be the equivalent to an early fetus.
Barely more than simple stem cell clusters that need a lot of nurture and dedicated development time before transitioning to actual Changeling status.
In fact that sounds like a valid reason to use "infants" for this task.
Risking a fully grown Changeling is deemed to risky, because even losing a single one is a great tragedy.
losing a few samples split off like a sperm donation is not something you lose any sleep over nor really expect more to come off it.
I'm not sure Odo is considered 'fully developed' by other changelings. His capabilities aren't (given that he still can't mimic human faces well - or is that something he consciously chooses to do, remembering his early days? Can't remember whether that's a fan theory or something actually said in the show), and both the female Founder and Laas (who never was in contact with the founders) treat him as someone with only a limited amount of life experience, much like a juvenile. But yeah, he's certainly further along than an infant Changeling.
Did we ever get any mention of how many the Founders reasonably expected back?
I don't think so. I only recall them saying they weren't expecting anyone back for another 300 years.
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