How much lead and silica sand in speaker stands?

Small ball bearings or small pea size stones work best, fill them to the top as you want them to be dampened as much as possible. I then use some poster board putty formed into small golf ball size then pressed hard between the speaker and stand this couples the two nicely.

Steel shot is a little more environmentally friendly than lead.  Supposedly dust-free kitty litter is also an option.
I suspect some blend of the possibilities that fills all cavities might work best.

Sand / Silica dust is a breathing Hazzard ... use ⚠ caution ... generally the more the better ...

When I see how a simple audio question turns into a virtue signaling contest I fear for our future. I really do. 

There's two things you're trying to do here. One is add mass so the stand doesn't rock as easily. This improves dynamics and bass extension. The other is to damp the ringing of the metal. This reduces glare and harshness and improves detail.  

Packed sand will come in good contact with the metal and do a good job of damping. But its not that massive. Lead is more dense and great for adding mass but its larger and won't control the ringing as well. That's why guys combine them. Best of both worlds. 

Works even better with a little mineral oil mixed in. Use just enough to get the sand damp. Mineral oil eliminates dust and helps the sand pack better. You want it packed, not loose. Mix lead in at the same time. 

Sand mixed with mineral oil is used in my turntable rack. Both the top and bottom shelves are sand boxes. https://systems.audiogon.com/systems/8367 

This rack was built some 20+ years ago. There's lead shot on top of the phono stage. The environment is still here, and the cancer seems to be under control. Oh and I haven't electrocuted myself. Yet. 

We now return you to our regularly scheduled irrational fear-mongering.

Lead shot now is expensive. I looked at the OP's system and if he is using the stands in the the photo, he needs quite a bit and sand can leak, over time, especially if you move them around. Been there, done both. I have found that small pebbles or pea gravel works very well with no need for sand and much cheaper. Very solid, no ringing and won't be a lead, sand, mineral oil mess if they need to be emptied for moving or to sell.