Answer
This will differ according to the size of the Musjid. If the Musjid is small
then it will not be permissible to walk across another person in Salaat at
all.
The Fuqahaa have calculated this to be approximately 335 square meters. If
the Musjid is larger than this estimated amount of 335 square meters then it
will be permissible to walk across another person in Salaat from a distance
of anything more than two complete Saffs from the place where the person in
Salaat is standing. (Ahsanul Fataawa vol.3 pg.409)
and Allah Ta’ala Knows Best
Mufti Ebrahim Desai