Impossible to say.
Everything you look at on the internet is downloaded to your PC, so it depends on what web sites you are looking at, and what you are looking at.
Text is small (size wise), images and graphics a bit bigger, and video is the largest of all.
So if you spent 6 hours looking at text based web sites you would download hardly any data, if you spent 6 hours watching videos on YouTube you would download quite a bit of data, probably gigabytes.
In fact if you watched a couple of minutes of HD video, that would probably download more data than spending 6 hours looking at text based sites.
The more tabs you have open the more data that will be downloaded, but the data from a web page is downloaded only once, so it would make no difference looking at two web sets from a single tab, or a web site each from 2 tabs.
It is not the number of tabs you have open but the number of web sites you visit. (and what you look at one those sites).