The card reader (esp cheap card readers) can simply read wrong sometimes. The reader does not "corrupt" or affect the card (it is reading, not writing), but it just may not always get the data transfer right.
It does not hurt the card. Put the card back in the camera, and the camera will be able to still see all those same pictures (that the reader missed), until you format it. If the camera can see the pictures OK, the card is not bad.
Formatting in the camera is important, to make the card empty for next time, but do be certain that your transfer to computer worked well before you format your images away. "See" all of those images are stored OK on computer (and then back those up elsewhere for safekeeping).
Sometimes simply trying the transfer a second time may go better. Reseat the card in the reader, and try again. Might be fine, if it ever worked.
Or try another card reader once (the camera can be plugged into the computer directly, being it's own card reader. A suitable cable came with the camera.) Down side is the camera has to be turned on, running on battery. And we can buy faster USB 3.0 card readers.
Next time, select a name-brand card reader (Lexar or Sandisk are good), instead of the cheapest no-name. Many of the cheap readers are good, but the reader is really the first thing to suspect. If they work, they work, but if they give trouble, try a different one.