This is such a situational question. What tier? what tank?
I can give some examples....
t10 I never ever use gold rounds unless:
1. E100 turret face
3. is7 at angle where penning ufp is easy
4. sometimes obj 263 when i cant hit lower plate ill spam gold at mantlet
5. vk4502b turret (dont even bother with lower plate or such, just gold his turret face)
6. russian mediums hull down, aim for flat cheeks next to gun
7. sometimes e75 (turret face) when i cant get good angle on lower plate
Thats all I can think of right now for tier 10s everything else you can deal with by proper aiming and weakspot knowledge without gold
Lower tiers is different. If im in a tank with low pen I will load gold for lots of things. Learn the armor values and weakspots. Youtube/tank inspector etc whenever you have a problem penning things at low tiers and youll learn 1 by 1 where to shoot and what you need pen wise to get through certain tanks. No one can give you a 100% certain guide that you will remember right away.
But to answer your question simply lower tiers I will usually keep around 15-20 gold rounds depending on ammo count. My tier 10s i only stock 5-10 depending on tank. Things like my batchat with low ammo I dont even have gold.