> Is a high or low reliability score better?
Low reliability is better (I know it sounds strange, read it as "unreliability").
> Is it best to try and focus aircraft by type at an airfield ie just fighter bombers / recon etc?
I do a mix of planes on normal airbases (6-7 per air command), plus slightly different build for VVS airbases (one per air command). When the regiments have 20 machines max, I do 3-4 ftr/fb + 2 TacB + 1 LB (or 3-4 ftr/fb + 1 TacB + 1 LB) per "normal" airbase (I do not use longer range LB there, like IL-4). On VVS I place 1 ftr/fb in night mode, 2 IL-4 LB in night mode, 1-2 recon, 1 transport. I think is is purely personal preference. Without order I'd be lost, others may not.
>Can someone explain what support/need 237/57 (39AV) means (I'm guessing 237 is the support available and 57 is the support needed but I am unsure what AV represents)
AV is how much support the planes alone need (so without the AA guns in the airbase). That way you know if you have too much aircraft in airbase. They seem to support around 120-150 planes per airbase. Air command HQ will not help to support the planes. 237 - support you have on hand, 57 - support you need (of which 39 is needed by planes).
> What are the consequences of not enough support at an airfield.
More operational losses, more damaged planes not repairing, regiments operating understrength (they do not fill to max), perhaps worse reaction on interception