Looking into Web Driver Tutorial:
Note: It is NOT the intention of this project to replace the HTTP Samplers included in JMeter. Rather it is meant to compliment them by measuring the end user load time.
So it is recommended to create the main load using JMeter's HTTP Request samplers and use one (or several) WebDriver Samplers to measure scripts execution time, rendering speed, etc.
Also pay attention to the quote from the 10 Minute Guide
Each browser consumes a significant amount of resources, and a limit should be placed on how many browsers the reader should create.
From experience, the number of browser (threads) that the reader creates should be limited by the following formula:
C = N + 1
where C = Number of Cores of the host running the test
and N = Number of Browser (threads)
eg, if the current reader's host has 4 cores, the formula would yield:
4 = 3 + 1
meaning that the script should have a MAXIMUM of 3 threads.
Also check out the memory footprint for a single browser accessing your application.
So theoretically you can kick off one browser per each CPU core after 1st and allocate around 1 GB of RAM for your browser (this may vary depending on how "heavy" your page is.
Check out The WebDriver Sampler: Your Top 10 Questions Answered for more details on Selenium and JMeter integration.