First thing to do is go to the heywhatsthat site and put your location in. This will determine what the theoretical range you should be getting is, as restricted by the terrain. Details are in this thread: ads-b-flight-tracking-f21/what-is-the-maximum-range-i-can-get-t35912.html
If you are using dump1090-mutability, then you can adjust the gain by editing /etc/defaults/dump1090-mutability. The valid values for gain are:
0.0 0.9 1.4 2.7 3.7 7.7 8.7 12.5 14.4 15.7 16.6 19.7 20.7 22.9 25.4 28.0 29.7 32.8 33.8 36.4 37.2 38.6 40.2 42.1 43.4 43.9 44.5 48.0 49.6
You can also use “max”, and “agc”. Max is the same as 49.6, and agc will in practice give a slightly higher gain than can be set manually.
It is really a matter of experimenting to find out the gain setting that works best for you. If you have the collectd stats logging running then it makes it easier to see the effect as you adjust things - you need to make a change and leave it running for a period to see what the effect is. The raw message count doesn’t necessarily give the best performance indicator, since it will depend on how busy your site is and where the planes are in relation to you - next to a busy airport might have different results to being somewhere where most aircraft are at range and high altitude.