"Should he have gotten a 40 slot panel instead of the 20"

Only you and the contract/drawings you approved can answer this.

If he followed the plan, and there was no discussion ahead of time about adding circuits later, then it's not his fault. Although if I was building it for myself, I would want to have some extra spaces so if it looked like it would be full, I would go for the larger panel from the get go.

What DB said, install a subpanel. Not a big deal.

You didn't post a picture of your actual installation, so it's hard to judge the situation.

I will say that I personally wouldn't want a new installation that was already full and left no room for expansion.

However, your panel will accept tandem breakers for 120v circuits. Unless all the 120v circuits are already on tandems, you still have expansion space to gain by replacing some standard single-pole breakers with tandems. And if you do get full up, as others have posted, you can have a subpanel (or two) installed.

Tadem circuit breakers the ugly step child of of breakers. What folks don't consider is the heat factor involved by removing 1 breaker and making it 2 in the same buss bar spot. This then doubles that heat factor onto the buss bar. Needdless to say not a good idea

I would only put in a load center with 40 (or more) breaker spots.

As an engineer - I really get PO'ed what some hack 'cheats' on an installation like that - just to save himself a couple of bucks in the bid.

I agree - get a subpanel.

Say what. On the bus bar? The bus bar heating is determined by only three things: the conductivity of the metal, the cross-sectional area, and the amount of current flowing. Whether there are tandem breakers installed or not isn't going to affect that. I'd believe your story a bit more if you were talking about heat disappation from the breakers themselves but that's also unimportant because of two things: first, there's very little heat generated by a breaker to begin with, and two the listing testing already takes that into account.

A far bigger issue are the fact that tandems/skinnys have issues that not all breakers are available in that format. Given the large numbers of AFCIs now required by codes and I've never seen a tandem/skinny AFCI or GFCI breaker, means that you may indeed run out of poles. Throw in a few larger 240V breakers and you've got more problems.

Anybody who installs a panel that doesn't have enough positions for the initially required circuits (and a spare position) is a fool.

I would request that they replace with another panel with 40 slots in it, rather than have to have a sub panel. My house was built in 1968 and came with fuses, rather than breakers. We had the panel changed out in 2009, and my electrician said, 40 slots was better to go with, for future expansion. Several of those extra slots are now in use, after a few years of having some new electrical added, and some changed to other locations, and also addition of other applicances, etc.

Depending on which version of the code you have, all 15/20A circuits serving outlets in bedrooms (old code) must have them or in the newer code all living areas. This includes both receptacles and lighting (or anything else) in these areas.

I see no reason why you can't install an EV charger, however.

Three spares is plenty if the rest of the house is properly wired (unless you're in California).