microstrip antenna gain range?

Rect. patch working at fundamental mode has directivity ~ 8dBi.
Simple estimation is following:

- you have 2 magnetic dipoles at radiating edges (2dBi + 3 dB)
- ground plane acts as mirror --> + 3dB

2+3+3 = 8 dBi

directivity simly varies with size of antenna.
You can have a look on small antennas fudamental limits regarding relations between antenna size and other parameters for a better view.

what is the maximum practical gain of a probe fed microstrip antenna ?

you mean a patch with coax feed through ground plane? (vs. inductive probe)

anybody knows what is the microstrip antenna gain of remote car keys?

i wonder how they fit a practically short microstrip antenna to radiate at 315/433mhz freq inside remote car keys .... any ideas?
(sorry might be out of topic already)

The gain of a microstip patch depends also on the substrate thickness (and dielecric constant), size of GND.

A small GND would broaden the beam significantly (a patch on a key holder is not a patch anymore - since the gnd is probably very small) so the gain is smaller

a thin patch does not exceed 5.5 dB (32 mil think on 5880)
a thick patch (air instead of dielectric) can indeed reach 8 dB