The “Generic” EQ setting is just that. It’s designed to accommodate any equalizer that REW isn’t specifically set up for. Parametric EQs that have been sold over the years can have as few as three filters, with some later models having up to 20, I think. As Bernard mentioned, you just activate the number of filters your particular equalizer has.
However, there’s a good chance you won’t find the “Generic” setting terribly useful. As you can see from the graphs at this post, equalizers don’t necessarily “resolve” filters the same way, for lack of a better word. Unless your equalizer happens to match the characteristics of REW’s generic setting, your actual results won’t turn out the way the program predicted. With equalizers not supported by REW, it’s best to just use the RTA feature, which allows you to tweak filters and see the results in real time.