Train from Bruges to Delft, then Delft to Amsterdam

The only reason to buy in advance is if you are using the Eurostar for part of the journey, and going from Bruges to Delft to Amsterdam, there is really no reason for that. Belgian and Dutch IC trains are frequent, and pretty much always the same price, and nobody gets reservations. Anymore than getting a reservation for 5:15 LIRR to Ronkonkoma.

Thank you!

Do these routes have enough trains/seats

You can always buy tickets at the station. It is not as if suddenly the railway says "we have sold enough tickets, so we stop". They never stop selling tickets. If a 1000 people turned up in Brugge one morning and wanted a ticket to Amsterdam the railways would sell 1000 tickets. If they all would try to get on the same train however not everyone would have a seat.

Tickets are for a route, not a train. So for the normal trains you can buy a ticket Brugge - Amsterdam, then on the way to Amsterdam get off in Delft to have a look around, and then later the same day hop on a train to Amsterdam.

This is different if you take a train with compulsory reservation. But that only applies to Eurostar services. Normal trains in Belgium and the Netherlands don't even take reservations.

If you are staying overnight in Delft I would not bother with getting a ticket at all. Just tap in/out with your credit card. The railways in the Netherlands (and in Belgium) are mass transit. More like a Metro system. Many mainline railways in the Netherlands have better service than most US subway systems.

I'm traveling in the Netherlands right now and recently spent about a month in Belgium. I've had to stand for a stop or two on several trains even though I wasn't traveling during rush hour. The comparison to a trip by subway is apt, including the dearth--or complete lack--of luggage racks except those located over the seats.

Belgium has a great deal on tickets for seniors, so I was careful always to select that option.

In the Netherlands I've just tapped in and out so far. However, there are sometimes discounts in Dutch rail tickets (I think not on all routes, and only during lower-demand periods) if you are willing to commit to a specific time block and buy your ticket at least the day before travel. The option will pop up if you search for a ticket on line. I've see discounts of €4.xx on the occasional short trip and at least once €9.xx on a longer trip. Tickets are otherwise expensive on a per-mile basis, so that discount could be helpful, especially for a family.