Retired Guerrilla explained it pretty well or do a search. There are lots of posts on them. Its basically putting as many plants in the room as possible and growing the main cola only. It is the most productive growing method. I haven't done it so I'm not the one to advise you on it.
Lots of work watering them but this also allows you to rotate the plants at the sides of the room/tent into the centre and vise versa.
5 x 5 is a decent size. You may lose some space around the outside for dehumidifiers, Humidifiers fans etc. But you should be aiming for a pound dry. This is easily achievable and less than a G per W. Once you have done that then you can aspire for larger numbers as you dial in your room and your system.
And ill reiterate, the HLG 550 will do your 5 x 5 tent esp so if you are rotating plants. If someone is insinuating that HLG are not trustworthy then they would be in the minority on this site. I'm no expert on HLG gear and maybe i'm wrong so lets tag someone who can help your HLG550 in a 5 x 5 tent dilemma and K rating choices for a veg and flower light (personally id look at a mixed spectrum) since Tim keeps going back to lights debates as he has a tendency to do.
@Stephenj37826 @CobKits