Too Lazy To Google

Two, plus a sliver of a third. Make the sliver larger if you're counting silver content (90% for the silver ones) only.

12.5g each, and 28.35g/ounce.

Problem here. Are you referring to uncirculated coins or slightly worn coins or excessively worn coins?

Fresh from the mint, silver halves have 0.3618 troy ounces of silver. So using that figure would mean 2.764 90% silver halves to make an ounce.

Alot of bullion dealers use a figure of 0.715 troy ounces of silver for worn coins per $1 face, so carrying that ratio/percentage over to a single half it gives 0.3575 troy ounces of silver.

Using 0.3575 as the amount of silver, it means you would need 2.797 90% silver halves to get an ounce. :)