How much sand can an engine apbsorb?
 

I know this is a stupid question: the answer is none, but here is the situation:

I was taking off the lower intake and, amongst other things, I found walnut shells, leaves, sand, that nice engine gunk and a lollipop stem sitting on my lower intake. So I scooped then brushed then used a vacuum cleaner then compressed air but when I finally pulled the intake, some sand trapped between the cylinder heads and the intake fell onto the rollers and a bit into the intake ports. I wiped and vacuumed 98% of this up. Should I be overly concerned about a few wayward grains of sand? Will the engine grind them up or it it going to seize immediatley?

Thats not a good situation bud :( ... no amount of sand in a motor is good but what do you mean by a few wayward grains ?
At very least and bare minimum i'd change the oil then run it no more than 3 or 4 min (no load, no reving) then pull the pan and clean it out throughly and change the oil and filter again.

If I pun my hand along the roller deck, every once in a while I'll feel a grain or two. Wont this all end up in the oil filter after a few minutes?

Most likely yeah ...

In all likelyhood a few grains of sand aren't gonna ruin your motor. if they would i'd be in big trouble cause i live near the desert and drive through it often ... and anybody who knows the desert knows that freekin sand will get into a submarine, you can't filter it all no matter how hard you try.

All it takes in one grain of sand to deteriorate a bearing surface or load surface and...uh oh.

With any luck the oil filtration system will catch it first.