I don't think there is such a limit which is not normal to any software. In the case of STEEM the specific challenge will be to scale to meet the ever changing demands of end users. @ura-soul has commented about this above. One of the technical challenges was the ever increasing size of the block_log (blockchain) I believe with the introduction of RocksDB that is addressed now. That being said extensive testing is needed to show the scalability of the blockchain when it is working in the full RPC mode.