How many times can data be read from a USB flash drive?

There is, for practical purposes, no read limit. There really isn't a write limit either, it's an erase limit. (And, if you've previously written to a block, you need to erase it to write new data to it.)

It isn't so simple to answer. When you write a file, new blocks are being written. The used blocks are marked as "dirty". So, if you wrote a 10KB file on a 1MB device, it is likely that the 10 KB file will be written all across the blocks in the 1MB device. Only when there are no more "clean" blocks, the flash controller will likely then erase "dirty" blocks.

Your flash drive will last significantly longer than 100,000 writes of the 10K file on a 1MB device.