One of the last few patches changed those details. You can now only have one defensive buff and one offensive buff from potions. If you use another of the same type it will overwrite the prior applied potion and its usage will be wasted. Some of the high end flasks count as both an offensive and defensive potion.
I used to have four potions going concurrently, like troll's blood, stoneshield, mongoose, etc. That doesn't work anymore. I've also not determined if scroll buffs count with the potions or can be applied separately. I know that food buffs are separate.