Thanks everyone so far for voting and your replies!
I think the baseline 4-5 PCs is probably close to the norm, but a lot of groups do have "extras" in the way of companions, hirelings, etc. For the most part, I would usually equate a number of these to add +1 members (or more) to the effective number of PCs in the party.
Since it seems to me from the evidence I've seen online and IRL discussions, most tables use feats, and roughly half or so use MCing to some degree or another.
With those things in mind, I am not surprised to see several DMs complain about how easy a solo BBEG is defeated if you use the default guidelines in 5E. Experienced DM will adjust accordingly, but novice DMs will be surprised at how easily the party dealt with "Monster X".
In our CoS game, the DM is novice and has only played for a bit less than a year. He runs the game well and we all have fun, but he is surprised how easily we defeat foes that he feared would be too much for us. This is due largely to the fact that myself and our normal DM are gamers-of-ol' and know how to min/max and exploit synergies--even without intending to do so sometimes (what can I say, it has become subconscious in some ways because in AD&D you more had to in order to survive--not so much in 5E normally).
While I understand WotC's desire to make the base game easy to play, and a lot of variant rules are offered, I wonder sometimes if they should design more for the middle (some feat use, some MC, some synergies, etc.) instead of the low-end or the high-end.
I think other than in some cases, it is part of the reason why the game loses some appeal after 10th level or so, maybe a bit higher. A while ago I worked on an L10 variant, but I am convinced now L12 is really the sweet spot for the end game. I'll be working on tweaking the classes, etc. but keeping them somewhat towards a lower-end of power, so that when feats and other features
are used, the balance of power between party and encounters is more even.
Anyway, please keep voting if you haven't already, and thanks for your input!