Pompei - anyone know the current situation regarding availability?

We were at Pompeii last week.
I'm assuming since you're taking the train, you'll be riding the Circumvesuviana train from Naples to Pompeii? If so, read on-

We arrived at Pompeii around 10:00 am, a little early to meet our 11:00 am Mondo shared tour guide, ( which I recommend.) we gave our cash to the guide, who walked up and immediately purchased our tickets at a secondary gate.
There's a carnival- type atmosphere in front of the main entrance, with crowds of folks getting off the train, and food and merchandise vendors all around. The lines to tickets at the main gate didn't seem very long.
Do not listen to the folks at the train station guiding you to an upstairs office to buy tickets. They are private sellers trying to sell their tours, not Pompeii employees.
I have not read about Pompeii ever selling out; the area is huge!
Leaving Pompeii, I must warn you to be prepared for a very crowded train, very hot, standing room only. It was actually painful, after being on our feet at Pompeii for several hours. I suggest you have plenty of water, and aren't hungry . The train ran late, and because of areas with only a single track, our train was delayed as it had to wait for other trains to clear the single track areas. The afternoon train took twice as long as the morning train.
I didn't look into the Campania Express Train, from Naples, which only runs four times a day, but I would recommend that, as it's less crowded and has air conditioning.
Pompeii is amazing, but as I think you know, coming from Rome will be a long and arduous day.
Good luck!

Do they restrict the number whom they allow to enter

The ruins area is 44 sq. Kms wide.

just buy 4 adult tickets online now.

What's the benefit of doing so?

The benefit of buying four adult tickets online now would be:

1. Guaranteed entry, without any concern of them running out of space. (Last year they were metering entry and not everyone was able to get in.)
2. Not having to wait in line at the ticket office. I don't mind waiting in line if it is 15 minutes or under, but I'd hate to have to wait in line for an hour. I'm not sure how long the line gets...

With regard to No. 2, it seems that online tickets are only sold in advance and not day-of. So if we get there an there is a long line, it won't be possible to bypass the line by purchasing online (as is often possible at other venues).

Last year they were metering entry and not everyone was able to get in

You mean because of Covid? Even if it happened in the morning, and I doubt it, The last Covid restrictions were dropped months ago.

Before the outbreak I doubt they have ever turned people away. As I wrote above the area is so big that the chance of "them running out of space" seems unlikely. At worst they may close the few houses with original stuff where most people crowd.

@Pat- Can you share more about how you booked with your Mondo tour guide? We are planning a day trip to Pompeii in August and would love to have our guide squared away ahead of time. Thanks!

Emifa, just go the Mondo's website under shared tours and sign up- it's easy. They will confirm with you when they have at least 6 people, which should not be an issue in the summer. We received our confirmation 4-5 weeks out, but some folks didn't get confirmations until the week before. It's a good tour and I can't imagine finding your way around Pompeii on your own. The guide has one of those mike and earpiece systems. The tour starts at 11am.
Have a great trip.

Thank you Pat!

I just did the Mondo guide today and would also highly recommend it if you can find an available date. Our guide was great and I really enjoyed being in a small group of 6. Our tour started at 11 am. We bought our ticket at the time of the tour and only waited in line for a minute or so. I don’t think there’s a need to purchase in advance.

Watch out for pick pockets on the trains. They work in teams. I had my camera swiped...and I even knew something or other was going on. They are slick and fast. An old man with a newspaper sat across from me...nearing the stop a very loud young couple arguing came up the aisle distracting me...just before the doors opened and I was starting to stand up the old man also stood up unfurling the newspaper towards my face which I avoided and when I stepped out the train door 3 seconds later my camera was gone...probably cut the strap with scissors.