I would just go ahead a taper that multiple times thought back a bit if I was you. Bigger dogs usually tend to be a LOT less energentic and quite often that includes sexually. EVERY dog is unique, mind you, but that's where the odds lay on that. The multiple aspect is also going to be GREATLY effected by the length of time you're in play, so to speak. It's true that the younger the dog, the more "active" they can be, just as the opposite is true.
If you'd said EXACTLY the same except were meeting a Jack Russell Terrier, I wouldn't even comment on that because those little fuckers are wound pretty tight, he might stick you 5 times in 5 hours, but a St Bernard is a different deal entirely, in my opinion, you'll be very very lucky to get 2 in that same 5 hours. Other opinions may vary.
All the more important for you to just be open to enjoying the experience without a whole lot of expectation.
Expect sex, expect to be excited, expect to be experiencing something you've wanted for a long time, and just start with that. Expect the things you KNOW are solid reality, and anything above that is a bonus, no? In short, don't overload the experience with expectations and take a chance that something you've presumably wanted for a long time comes out as disappointing because it didn't meet expectations. Go in without the weight, if that makes sense?