I rather hope that the reality of Founder existence is more alien and interesting than just a fixed number of shapeshifting people occasionally taking a swim in the same pool.
Staying almost humanly individualistic may be a characteristic of immature Changelings only, and Odo may be even exceptionally atavistic for having grown among Solids; others of his age might already have learned to abandon individuality more completely.
Perhaps the Founders are indeed just one entity, and when it has "children", it in actual fact just detaches some autonomous parts of itself to gather new experiences, and then return so that those experiences can be fully assimilated. A bit like how when Odo shatters, his fragments "instinctively" flow back into a whole (say, when he's a glass in "Vortex"), but Odo himself strongly believes that his fragments do not possess his full abilities and will at the very least lose their shape when detached for a long time... The Hundred may be fragments of another sort, on a higher level.
And who knows, perhaps the great planet-sized pool they called the Link is just another fragment of a still higher-level entity?
Timo Saloniemi