How many OP_RETURN transactions in the Blockchain would be too many? I realize this question may not have an answer and therefore might not be suitable for SE, but I'm very curious about OP_RETURN transactions.

OP_RETURN transactions appear to be the accepted mechanism for inserting metadata into the Blockchain. Considering the innumerable uses for metadata (contracts, colored coins, proof of ownership...), I foresee more and more non-monetary transactions filling up the Blockchain. 

Will that be bad?
It is reasonable solution. The main problem is not in the size of blockcain itself. The problem is in growing the number of unspent outputs. OP_RETURN outputs do not increase size of Utxo database