When you are using a wifi adapter in monitor mode, you are not associated to an access point. It will simply listen on the configured channel(s) and capture any traffic it understands whether that is 802.11b, 802.11g, or 802.11n.
If you are associated to an access point, then you are running in promiscuous mode, and then you are generally only going to capture traffic to/from your station plus any broadcast/multicast/management frames.
Now the flip side is that you need to be fairly close to the source of any traffic running at 300Mbps to be able to capture it. The further you get away from the source, the slower the data rate of any usable data you will capture.
This is one of the "tricks" in analyzing any wireless data and when using wireless tools. You need to understand that the data is presented as it is seen from the current location at the current time. I have used tools that indicate there were hundreds of 802.11b clients around me, but that is because the software tool was only capturing traffic from those clients when they were transmitting at 1/2Mbps and not at higher speeds due to distance/obstructions/interference/noise.