Subj. errr... it's 43, right?

Also, don't you think it's strange (from the gameplay perspective) that the better you play, the higher chance you have your buildings to last ? I mean, when you have a good run, you don't get your buildings wrecked anyway, but a sloppy player gets punished even more. And overall, it's still throwing a dice (I had a very poor ran, when I got my buildings sustain the damage several times in a row). Kinda strange mechanic in overall not prone to tempting the fate game.

I believe it's 40: the default is 15 (on normal mode, at least) and the tooltip suggests that you can increase it by a maximum of 25. If pilot skills don't count against the limit then you can probably get up to 49 by all three pilots having the '+3 grid defence' skill.

As you say; it's a bit strange that the main way to raise it is by preventing damage to your buildings in the first place, but you can't really rely on it anyway - I had mine up to 25 on my best run and still never saw it trigger.

I think the main benefit from it would be in the final mission where it's extremely difficult to prevent all grid damage - if you've done really well up to that point then it gives you a bit more security.