If you read the YongNuo manual is says that up to 180 units can be fired ... perhaps that is the number they had available for the test ....anyway ....
The reason to use the pop-up is that it is the simplest and cheapest route .... must admit I have something to fire multiple flashes but never used it so far ... I rarely use flash anyway being lazy.
I wonder Isabel if your aversion to the pop-up comes from the downplaying of it firstly as a bad tool and ineffective ... when these days it is neither when used in an appropriate way.
Note I said to hide the subject from the pop-up if you like ... it would avoid 'red-eye' effects ... but when it is a 'still life' there are no eyes to get red.
Much as the pop-up is good as a syncro sunlight tool to lighten shadows cast by the sun on our subject what we are doing is simply using it as a 'Fill' light in the studio terms to avoid the harsh shadows from the 'key' light. This is a technical requirement of firstly film and these days the capability of the sensor.
Frankly I detest and consider pretty bad the common approach to studio lighting of using a couple of flash units with umbrellas, flat lighting with light splashed all around the place .... but then I was started in studio lighting with spot light[s] and flood fill lights. Ratio one to two was the allocation as a student.
In using the pop-up originally I covered the light with a piece of card but subsequently when I felt it was too strong I covered half with my finger for a better key/fill balance.
You are well away with your YN flashes as you can turn the base to face the camera's pop-up and unless you are going to do portraits the light is probably quite suitable .... though as you get past first base you could probaly make some cardboard tubes to stop the light spraying all over the place but going where you want it to go and there are also 'flags', hanging curtains/shades and all sorts of tricks mentioned in that book I suggested as well as lots of other places.
Read up about using Mode S2 which is supposed to supress pre-flashes the camera may admit for focusing etc and only fires for the photo
This is the simple cardboard tube with a hole cut in it so it worked as both key 'spot' light and background light.
While it has been suggested that you get a stand for your flash units another useful gadget is a boom support for the flash ... I am a DIYerNut so I made it myself before I invested in proper lighting stands.
The 'J' stands for John but really I am jcuknz and if you did a Google search you would find page after page of references to the guy ... but when I used my name I think it was around page five before I found something I could tie in to myself and earlier found a professional photographer of my name in the UK ... so better to use jcuknz to avoid misunderstandings
Also shown in photos are two of my early "optical triggers" which are needed if using flashguns without built-in variety which you fortunately do not need unless you pick up a s/h unit for a third unit etc. I worked it out that buying a 'dumb flash' and a trigger cost approaching what you pay for the YN but I have mine from the past.