Thanks Kirt for your answer, but does it really
depend on the instrument ?

An explanation
is given on

http://ku-prism.org/video/JohnPaden/network_analyzer_oper.htm
http://ku-prism.org/update/SummerSeminar/MrPaden.html
According to this, the number of points is determined
by the maximum length of the impulse response and
the bandwidth of measurement:

number of points
= length of impulse response * bandwidth of measurement

First of all I don't understand this simple formula,
secondly how do I know the impulse response of my
DUT ? Generally, that's unknown I would say.

The necessary IF bandwidth is determined by
the desired dynamic range (SNR).

Finally,
the sweep rate is calculated by the bandwidth, number
of points and group delay through the IF BW-filter
(approximately the reciprocal value of the chosen
IF BW). The sweep rate, together with the length
of the impulse response results in the change in
frequency during the impulse response, which must
be lower than the IF bandwidth ("the sweep time
is always okay if you are properly sampling the
frequency domain!").