Minimum required at destination is alternate fuel plus final reserve. Route reserve may be used during flight (for ex for weather avoidance,different FL,etc..) .
Arriving at alternate you will have the final reserve remaining on board.According with regulations ,this is the fuel at which you should declare an emergency- MAYDAY.
If you'll have to hold at alternate,you are in for some trouble.You should've anticipated this well before arriving to alternate and plan for alternate scenario.For ex don't hold at destination untill you arrive at minimum fuel OHD if the alternate presents some chances of poor weather also.Or plan for 2 alternates if weather seems to be a problem around ETA.
In any case you should avoid putting yourself into a corner by any means.