Nope they cannot. Island building is done on existing low tide elevation which are maybe couple of feet under water. So you only need a little bit of soil to build them up. Within a few meters from the island, you are no longer dealing with shallow sea floor but several 100 meter to km deep ocean floor. No one can build on that.
Practically speaking China can keep expanding much more than they have, because they have not built on the entire low tide elevation.
Example here of Mischief reef:
View attachment 169768
They can expand on the grey low tide areas in the image. But they cannot expand on the deep ocean seen in the image as deep blue areas.
But Vietnam has much more islets they control and can do island building if they want to utilize their entire controlled islets. China only controls 7 features and has already built most of it.
View attachment 169769
Most of the Map is Vietnamese flag. PRC flag is only on 7 features.