VobsubMuxer can handle up to 99 sublitles.
Dvd format restict to have 32 subtitle streams in total for each vobsub file.
So it is always good practice to respect that limit. (at max 32 languages in one vobsub file)

From the version 1.9 and up you can load all your vobsub files that you want at once.
The older versions had a 8 file limitation
The same is valid with the command line too.

Older version of VobsubMuxer can handle up to 8 multilanguage vobsub files that containing less that 32 subtitle streams in total.

Attention : VobsubMuxer handles correctly only Tx2Vobsub generated streams !!!

Example1 :
Vobsub1.idx-Vobsub1.sub [one language]
Vobsub2.idx-Vobsub2.sub [one language]
Vobsub3.idx-Vobsub3.sub [one language]
Vobsub4.idx-Vobsub4.sub [one language] +

---

Vobsub1.Muxed.idx-Vobsub1.Muxed.sub [four language pack]


Example2 :
Vobsub1.idx-Vobsub1.sub [six language]
Vobsub2.idx-Vobsub2.sub [three language]
Vobsub3.idx-Vobsub3.sub [one language]
Vobsub4.idx-Vobsub4.sub [one language] +

---

Vobsub1.Muxed.idx-Vobsub1.Muxed.sub [eleven languages]

Questions :
1) I want to add 12 single language vobsubs to one how can i mux them since VobsubMuxer loads up to 8 single vobsubs ?

In the 1.9 version and up you can do it directly by loading all your subtitles at once.
In the older versions you have to proceed like this : This is really easy ... you have to do 8 per time. step1 :
Vobsub1.idx-Vobsub1.sub [one language]
Vobsub2.idx-Vobsub2.sub [one language]
Vobsub3.idx-Vobsub3.sub [one language]
Vobsub4.idx-Vobsub4.sub [one language]
Vobsub5.idx-Vobsub5.sub [one language]
Vobsub6.idx-Vobsub6.sub [one language]
Vobsub7.idx-Vobsub7.sub [one language]
Vobsub8.idx-Vobsub8.sub [one language] +

---

joined1.idx-joined1.sub [eight languages]

step2 :
Vobsub9.idx-Vobsub1.sub [one language]
Vobsub10.idx-Vobsub2.sub [one language]
Vobsub11.idx-Vobsub3.sub [one language]
Vobsub12.idx-Vobsub4.sub [one language] +

---

joined2.idx-joined2.sub [four languages]

step3 :
joined1.idx-joined1.sub [eight language]
joined2.idx-joined2.sub [four language] +

---

joinedfinal.idx-joinedfinal.sub [twelve languages]

2) What is the maximum limit of the subtitles that i can add in one vobsub subtitle file ?
You can add up to 99 languages but this render the vobsub subtitle un-producable in the most players (hardware and software onces).
So it is always good practice to use up to 32 subtitles per vobsub file.