do I have 9TB transfer quota for NLB if I bought 3 $20 VPS?

I'm confused about the NodeBalancer plan. What's the maximum traffic allowed for it?

I'm planning to setup around 7 servers for my mobile app, the setup probably looks like this:

NLB x 1

app server x 2 (running apache+php)

memcached server x 2

mysql server x 2

except NLB, all other servers should just use private network and won't communicate with public internet. So if all other 6 servers are from the $20 plan (3TB for each), I'll have 21TB traffic allowed?

I've heard all the traffic quota for my linodes will be combined as a "pool", does it mean I don't really care the traffic volume on individual machine as long as the total internet traffic from all my servers doesn't pass the total quota?

Thanks.