Was this made with the intention of being a single standalone film or were more entries planned from day 1?

You might already know Spielberg and Lucas were inspired by the old matinee serials they used to watch as kids, so they probably thought there was a possibility of future adventures in the same vein involving the same characters. But having watched and read all the "making of" features back in the day, I don't recall seeing anything to suggest they had planned any of the sequels from the start. I would say they produced this as a standalone movie and only thought about a sequel when it turned out to be a blockbuster hit.

The original title is one clue. When it was first released it was simply titled Raiders of the Lost Ark.
Indiana Jones and the... was added for a 1999 video release, to make it consistent with the sequels.
https://i.pinimg.com/736x/8d/71/42/8d7142404d9ab98323282727ad1daa72---s-movieposter.jpg

I highly doubt it was originally conceived as a series. That changed when it received overwhelmingly positive response from both critics and audiences.

This came out after Jaws 2 and Empire Strikes Back were released. Sequel talk was definitely on the table. But I would say this was made as a stand alone film. It honestly could have totally flopped. There's a lot of cheesy outdated elements to Indiana Jones which really shouldn't work. But the highly charismatic Ford and incredible stunt craft made it a home run.