Our table has one column family which has a TTL of 15day. So rows fall at a consistent basis. We are seeing that the no of regions are going up. Some how the regions are not getting re-used. We are currently with over 41k regions of which 19k are empty. Any pointers as to why regions are not getting reused. Our row key design is similar to what you mentioned. 2 digit salt (0 to 63) followed by hashcode of reverse time stamp in seconds, followed by a service id and finally a counter.
Our cluster is 41 nodes and we are writing rows at a rate of 4k to 6k Tps. The average row size is about 35kb.