Thats a bit like asking how long is a piece of string.
Performance will degrade when the combination of CPU, Disk and RAM cannot meet the throughput your system requires
With 16GB RAM and 16 cores I would expect that disk IO may become an issue first but without a full examination of your system it is almost impossible to answer you question
There really is no magic database size where performance will go downhill. If your database has 5 users and they only issue queries which only use 3 records per query, then the database can get up to 1 PB and it won’t have any problems.
It is all about the amount of data that you need to store in the servers cache.
Way more information would be required before we can give you a better answer.