i do want to know, what is the limit to enemies per stage
There is no practical limit for enemies "per stage", only a sprite limit "per screen" (8 sprites per scanline) that is why I've mentioned the NES color limits. It is the amount of colors on screen that determine the sprite count per screen, and thus influence everything in the stage. Every 3(+Transparency) colors equals a sprite overlay, as with Rockman. So usually enemies are kept at 3+T colors (one sprite) to make a lot of enemies appear, whereas more than 3+T colors (two sprites) are used whenever only one or two enemies appear.
The background will also influence how many enemies will appear. As there are 16 colors possible on each screen, enemies can't be adding too many colors. To elaborate, after assigning 4 palettes to the tiles, how many colors you still have to spare in order to reach 16, will determine how the enemies are colored and which enemies will appear closely together. Usually Rockman6, which brings out the most of its backgrounds, uses only one additional color for enemies that appear on screen. If it wishes to do more, it'll remove one palette from the tiles and start using enemies that add two additional colors on screen. Every other color the stage enemies use are often identical to the background, or the main character.
Notice how these limits interact: if you want more colors for one enemy, you need a sprite overlaying, thus lessening the amount of enemies on screen. Likewise, if your one enemy or groups of enemies introduce additional colors, you must decrease the colors in the background to compensate.
The inverse is also true, if you want more colorful backgrounds, you must make the enemies less colorful. Which means they'll often use only one sprite overlay, and thus will appear in abundance.
I mention these limits not to needlessly restrict you, but because they serve as nice simple guidelines to base everything on. By keeping them in mind, the NES look, NES feel, and NES gameplay balance should come naturally.