A myofiber is an individual cell. This is an important concept that can't be overstressed. A given myofiber may contain millions of myofibrils, all in register and lying parallel to each other. Here's a higher magnification image from the same slide, again cut in cross section. In this image the separation of the fibers from one another by the endomysium is very distinct.
Small capillaries run in the spaces between the myofibers.
Muscle has a very high demand for oxygen, as you would expect. There are nuclei in the myofibers and there are nuclei between the fibers. The ones between myofibers are those of fibroblasts and the mural cells of the capillaries. Note carefully the position of the nuclei in the myofibers. The myofibers are enormous single cells.