There is nothing wrong with having many Salagrama Perumals with you. It is not necessary that Thirumanjanam (holy bath) must be performed for all Perumals on the same day, or every single day. You can perform Thirumanjanam by rotation, for two or three Perumals at a time. You can keep them within your home. Alternatively, you can give them to those who perform Tiruvaradhanam. Do not immerse them in the river.