One theory behind the 7th injector was that it was able to add fuel on demand, and only on demand. That is, it allowed you to run 6 smaller injectors for good low demand manners, then have that 7th available to handle WOT blasts.
Sounds good, right?
In practice, some folks used the 7th injector as a band-aid for their 6 smaller injectors being too small - in order to avoid the expense of purchasing 6 new, larger injectors, and the hassle of having to install them.
Bottom line - today's better chips are very capable of running very big injectors for good low demand manners, with plenty of fuel for WOT blasts.
Perhaps in some cases the 7th injector was also a band-aid for a weak OEM fuel pump, a clogged fuel filter, kinked fuel hoses or lines, insufficient voltage to the pump, dirty injectors, and the like. Given the knowledge and equipment we have, none of these "should" happen nowadays.
I know some folks who run 7 or more injectors quite successfully. However, to me, they always seem like a legacy from the past. Expensive, and with their own set of problems.
Good luck!!!