We need to add 20 new nodes to an existing 13 node cluster. Are there any conditions on how many new nodes we can add a time. Or can we add all the 20 nodes together without any impacts?
You can add 20 nodes at once without any impact. However, for the new nodes to participate in queries, you will need to rebalance the data, which could have considerable impact on system performance.
How much data is in Vertica, and what version are you running? I would recommend upgrading to latest 9.2.x version to take advantage of catalog performance improvements. It would be quicker to upgrade the existing 13 nodes before adding new nodes.
There is a lot to read and understand here.
But for the most part its a simple operation of adding the nodes and re-balancing.