Paulo,
I don't know how to estimate the number you asked for. There are something
like 50,000 blind and visually-impaired K-12 students in the US and many
many more in other countries, including much larger numbers in China and
India.
You might want to look at the archives of the international Blindmath list
to get a sense of the need.
http://nfbnet.org/pipermail/blindmath_nfbnet.org/2011-October/date.html
However I should warn you that you could start a flame war if you were to
post your query to blindness organizations as it would be considered as
quite insulting.
You can also search on the term "math" in the profiles posted in conjuction
with the recent Braille21 international conference:
http://www.braille21.net/en/braille-ambassadors
Note that this question is somewhat of a chicken and egg question. While
there are many braille readers around the world who would love to have
better access to advanced technical materials such as those on arXiv, there
are likely a much greater number of capable blind students who've dropped
out of math, science, and engineering courses because they could not get
accurate and timely braille transcriptions of needed materials. (Here in
the US it costs upward of $5000 to transcribe a single technical book to
braille.
http://www.atpc.net/index.php?Page=BFee&Action=SetToDefault )
Many college professors prepare course materials in LaTeX and many
braille-using students are reduced to reading LaTeX source rather than
braille math. I know that as a sighted person with a strong background in
math I find it very difficult to come to a good mathematical understanding
of math expressions in unrendered LaTeX so I can only imagine how tedious it
must be for a braille reader.
The fundamental point is not whether access to arXiv per se is needed but
that any solution to that problem would likely be a solution to the more
general problem of providing better access to technical materials. You can
read more about the general problem here.
http://www.access2science.com/
Susan Jolly
P.S. Paulo if you are interested in helping with this problem I suggest you
start by contacting your UC Berkeley Disabled Students' Program or find out
what your own department is doing in this area.
http://dsp.berkeley.edu/
