1. Upper screen says: 500 conscripts companies [available, +how many more will come]
2. Military Recruitment Mode allows you recruit units like brigades, ships, regiments and put them on map. All numbers of available units are finite (eg. 3 available infantry divisions, 2 infantry divisions on map) - you can use all available units and no more (unless technology, reforms, options, events give your nation more)
3. F2 Military Production screen allows to purchase replacements (made of those conscripts from point 1) into elements that can be used by depleted units (from point 2).
Which point is your question?
"I have 1140 missing hits, say 500 companies of conscripts, but 0 ...none...available RESERVES. How do I replace the RESERVES so I can use them AS replacements?"
To purchase reserves you need conscript companies AND/OR horses, war supplies, money.
If you have 1140 missing hits, 500 conscript companies, 0 money and 0 war supplies - you can not buy replacements to be in reserve.
Meteoryt-like user. Strikes and disappears.