Are they regularly spaced? All in the same row of siding? How big are they?

It sounds like caps for insulation holes, but without the above info, it could be anything.

Are they louvered vents or just plugs?

My house had cellulose blown in my first summer owning it, back in '88--the holes are covered with plastic caps. I bought some louvered ones to replace them, as I read that some ventilation was needed, but I never put them all in.

Should I replace them all with louvered ones?

Vents can allow moisture to escape from the wall to the dry outside air (if the wall is wet) but they can also allow moisture from humid air and rain to get into the wall. Unless they are pencil sized and angled upward under the lap of clapboard siding I wouldn't recommend using them. The 3/4" to 2" sized vents are fine in exterior soffits.

Yes, they seem pretty evenly spaced on the same row of siding. They appear to be 2" or maybe a little bigger and yes, they are louvered.

Definitely sound like holes for insulation then...but I thought there should be more than one row per floor, as mine are done?

My house is from 1908, so I assume it's balloon-framed, but two rows each floor to account for fire-blocking?

"My house is from 1908, so I assume it's balloon-framed, but two rows each floor to account for fire-blocking?"

There may not be any blocking.
That was one of the overall weaknesses of balloon framing.
Many places had chimneys from foundation to roof in every exterior wall.

Fire blocking, when used, normally occurs at the ceiling line.

Thanks brickeyee and Ren, I thought I'd seen it in some old houses on tv about half-way up...wish I had an infrared camera to see inside my walls, because I think there are spots missing the insulation! If I knew where, I could fill those spots myself...but I can't afford to buy one.