T5 is nice, though S4 is decent as most armored units tend to have T3 or 4. Even though T5 and multi-wound, their 5+ regen is not helping vs KB and easily erased from any flaming attacks. Remember that if someone shoots flaming arrows, it only nuetralizes your regen for the rest of the shooting phase. You still get it in CC, unless a unit before you has flaming on their CC weapons. Same goes for magic. If they use flaming, your regen won't help the rest of the magic phase (if more non-flaming spell derived hits aren't dispelled) but unless they shoot you first with flaming arrows, you are good to go with regen that phase.
There are a number of uses that warrant 8. They work very well with Mortis Engines, which will increase their regen to 4+. A Crypt Scavengers Battlescroll and the Garden of Morr special rules are crazy for them (+1 regen, rerolling missed saves if within 6" of the graveyard!). But I mainly use them to screen the Mortis Engine, Coven Throne, or most often, the Terrorbat! If cannon fire hits the CH's, it needs to make it through two ranks of multi-wnd regening monstrous infantry in order to hit whats on the other side. If any rank doesn't die, either to a low wounds roll, or saves enough to not get killed, the cannon fire stops. 8, 4x2, is enough to cover most if not all the length of your TG. If you take 2 TG's, take 2 units of CH's to screen both. March them up (I usually have my Lords unit in between them) and launch your TG's ahead to liquify enemy units when in range. Or protect your ME's which will increase their Regen. Its a nice strategy combo, but it does have weakness (Fire...bad....and mass shooting).
So minimum, 8 models. Ideally, 16

Don't forget, you can use 40x40 bases as unit fillers with gravestones, withered trees, etc. I plan to use the extra CH stuff from my box that I made Vargeists with to show them coming up out of the ground, as I don't have the full models on those bases, but it works!