I'm no expert on the subject, but according to this guide ->
https://www.growweedeasy.com/manifold then there should be no point of diminishing returns. It really depends on how long you want to make the vegetative period and whether you have the space for it or if you are growing indoors or outdoors.
If I remember correctly, then the guide gives advice to not go above 16 colas unless you are growing outside, as 32 colas will take up a lot of space, but theoretically you can do whatever you want if you have the space for it.
On a side node, looking at your picture, it does not look like you are actually mainlining. A mainline requires that you only have 2 branches coming off the main stem. It looks like you have four?! The basic idea behind the mainline/manifold is that you want the plant to distribute water and nutrients equally to all the branches and you do that by creating a manifold.
Here is an example of what I mean.
View attachment 2492510
Hope this helps.