My heatsink launcher works only once after restock. What is their typical capacity? How to tell how much left?
Full capacity is 4. 1 loaded, 3 in reserve.
You can only tell how many heat sinks you have if the launcher is active in a weapon group, which will cause it to display its ammo on the hud just like a projectile weapon or missile launcher does. When I'm flying a ship with a launcher, I put it as secondary fire in a later weapon group, which I switch to when I need to check how many I have. (I have a button bound for firing them directly from my Eagle days.)
There's a long-standing bug (since alpha?) where Heat Sink Launchers lose their ammo for no apparent reason when supercruising. Generally it doesn't wipe out all your ammo, though, it just deletes some of it. It's supposedly fixed in the next patch, but I've heard that one before.