Here is some more info for you:
Total plugins: I've seen recommendations for as little as 5, or as liberal as less than 20. Your plugins are up to you and what's important to your site.
Note: When P3 is installed, even if it's not active, it can count as 1 plugin if you're using the must-use plugin loader. This is okay ... see the "Is this always running?" question in the help docs for more info.
Plugin load time: Lower is better. This represents a delay for the end user. The higher this number is, the longer they are waiting. Some delays (10ms) won't be noticeable. Other delays (100-200ms) will feel noticeable.
Here's more info from 2006 from Amazon that showed 100ms of latency = 20% loss in sales.
Plugin impact: Lower is better. If your plugin impact is 1% then only 1% of the time WordPress is taking to create your page before it is sent to the browser is spent in plugins. If your plugin impact is 80% then WordPress is spending 80% of its time in plugins.
If this number is higher than you think it should be, and you have plugins you can turn off, try turning them off and bringing this number down. It should bring your site's load time down, too.
MySQL Queries: Fewer is better. A stock WordPress install is ~18-25. If you're seeing a much higher number here (like 150) then you may want to start disabling plugins until you may want to investigate further.