I've decided to count my chickens long before they

I've decided to count my chickens long before they hatch. I read somewhere that investing in a small business C-corp for the long-term would mean half of your gains will be tax free when sold. Is this accurate for BLTA? I want to say it was something like 5 years minimum. And if so, when you sell shares, do they count the first shares you bought or the latest? i.e. Let's say I bought 100k shares 10 years ago and 100k shares 5 days ago. Then let's say you sell 50k shares, for tax purposes, do they consider that you're selling those old shares or the new shares? Just curious...something I have yet to encounter in my short time as an investor.