You can sometimes buy add ons to call recording equipment which can be scripted to allow consistency of checking.
Each QC process would be different for each company, consider what is important to you in the call, greetings, goodbye process, upsells, technical contents, tone of voice, sounding interesting, use of customers name, offer of further help, did they go that extra mile etc etc.
You can use point scoring for each part to produce meaningful results.
If you consider how long it takes to review a call, say 10 mins plus write up time. Take the time the reviewer has available, say 1 day a week. 7 hours = 42 calls a day less say 6 for non productive time. You can aim higher or lower if your resources allow. This could lead towards your percentage that can be evaluated rather than how many you want to evaluate.
You may want a higher % value for new staff or staff who have not performed as expected and lower % figures for your best staff.