No, it really, really doesn't depend on number of games
I have 422 GOG Games, all installed with Offline Installers.
I haven't bought more than a couple at a time, so first install not affected by Total number

For the most part patches are fine, after launch, no need to even install game again (exceptions exist).
Periodically I grab the latest full installs of games I already have patched several times, in case I need to do a fresh install.

I know the typical reasons for using such downloaders, from times of yore, and even today being a user of Pale Moon (for it's superior extension support), which is based on older Firefox code, it's browser is still restricted to 6 simultaneous downloads.

I use Grabit (Flasget Fork), and it can use pretty much every downloader ever made with it.
A common issue I have seen is it can download the bins, but names them all wrong something like
install-# where #=bin number minus 1

The issue reported seems to be same one I see with Grabit, but that tool can grab all links, and gives me the list of links, so I can select the bins, even though it gets name wrong, and I can't get it to rename them. I use Advanced Renamer as workaround, and remake bin name from the Exe name.
Easy Fix, with real extensions.
Firefox extensions are crippled feeble imitations of the entire reason I first switched to Firefox.
So Pale Moon will stay my main Browser.

Firefox is a mere backup now for me. only slightly better than Google Chrome.

Up to date Fiirefox has no need of 3rd Party downloaders.
Only today, I've been downloading multiple huge installers, simultaneously.
Maxing out my full 200 Megbit = 25 MB/s bandwidth.
Even Pale Moon will max out my bandwidth, but won't download more than 6 files at once.

Note GOG bin limit is 4GB (installer is 32-bit?) but that limits corrupt downloads to a 4GB chunks.

These were all being simultaneously downloaded
Cyberpunk = 27 Bins
Horizon ZD = 18 Bins
Chernobylite = 9 Bins
Hellblade = 4 Bins

There were some single file ones as well, but I ran out of updates to download.
So never hit a theoretical max number of simultaneous downloads, if it exists

All downloading simultaneously, now I may have gone overboard as I see now, writng this that, some failed, but I've had issues resuming such fails with Pale Moon + Downloaders.

Firefox's just resumed the 12 or so I see had failed, and it seems they had actually completed, and just failed to do the final step moving the "part" to the bin. They all instantly completed when resumed.

Regardless With both Browsers I was Maxing out my faIrly high speed Bandwidth.
Only the number of simultaneous downloads was different.

I used Firefox from v1 to v56, only ditching it when it became a clone of chrome, and ditched my must have extensions, so I'm no fangirl of the modern Chrome Clone, but give it credit when it earns it.

Unless you're using an older fork, their is NO NEED for a downloader at all.
Even that's only to download all files at once, just one big file will max out my bandwidth.

No need to split files into chunks, as a workaround, to max your unused bandwidth.
That's the entire reason why Downloaders existed. in the bad old days.
All games are far to big these days for Store Clients to limit your bandwidth
Maybe for Gigibit speeds, but I'm using well above the average download speeds.
I was still watching YT vids while files download, resource allocation of Windows is much better these days as well.

So if you use a modern Browser Ditch your Downloader.
If you're on an older fork look for Grabit, or other Flasget clone to get the manual link selection feature.
Also Note, I even had Flasget working, only switched because Grabit had workarounds built in.
So you can spot the bins, via backend names they show up as with Grabit/Flashget.

After yesterday, my testing was a success
I'll just use Firefox in future for very Large Simultaneous downloads like GOG offline Installers.
Pale Moon for day to day use, due to the QOL (Quality Of Life) extensions, I must have.

{Edit: Corrections)
Names mix up, It was late, and I was woken about 02:00 to deal with a loss of water pressure (Fixed by water company now) didn't do my usual double checks, substance of post is correct.

Grabit = Pale Moon fork of Flashget (The Downloader Manager)
GetEmAll = Pale Moon Fork of DownThemAll (Downloader I use)

The bins look like this in Grabit, (right click select "Grabit All") (Note en1 = language I guess
...downloads/a_plague_tale_requiem/en1installer0

Each needs renaming to <SETUP EXE NAME>-<Number+1>.bin
So for example given
Use
setup_a_plague_tale_requiem_20221013_1051_(64bit)_(59469).exe
As template to rename the Bins
en1installer0 = setup_a_plague_tale_requiem_20221013_1051_(64bit)_(59469)-1.bin
And so on.

Note: It's not GOG's problem that our chosen downloader is not finding the files, or renaming them.
Browsers work fine without using any Downloaders.

Why does GOG do it this way?
Seems that language choice is the reason, installers have same name, whichever language is used, but the actual files depend on your chosen language.
I've found Workaround solutions, and offer them as the solution, simply because I've dealt with this for a long while.

As I stated, I just wont bother to do this in future, and I only used Downloaders with big Installers like Cyberpunk, for 7 Bin Installers, or less Pale Moon is fine start with last BIN usually smaller than 4GB, then EXE, then other bins, by time I get to BIN-6 that last one is usually done.

I'll just use latest Firefox for larger ones now. No downloader required NOW.
If for some reason that won't work out for your Firefox Fork Flashget (or Grabit) is the way to go.