how many nodes can be add in Engineer's Toolset Network Performance Monitor

In the hundreds. The limitation of the DB is actually determined by polling intervals, how long stats are kept(esp. detailed stats) and how often the DB is compacted.

The Engineer's Toolset is not intended for long-term monitoring of networks with more than 500 network elements and has a 2GB limit on stored data. If you have a larger network and/or require data retention longer than 30 days, we highly recommend upgrading to Orion Network Performance Monitor.