Sharding the counters would help, but with MVCC the typical solution is a delta table containing +1 and -1 records that is periodically compacted into the main

Sharding the counters would help, but with MVCC the typical solution is a delta table containing +1 and -1 records that is periodically compacted into the main count. With some cleverness about how to perform the compaction it's possible to make that very efficient.