hi friends
1-How much memory error is allowed and is standard?Is it related to the duration of the working rig?
2-How much each riser is consumed(Estimated)?Does it have a relationship with the card??
3-What is Specifications of stable rig?
4-bios modding Is that just to set 1500 memory strap to 1750 and upper?
It sounds like English might not be your primary language, but I'll try to understand.
1) You shouldn't have any memory errors. 4 GB is the standard amount of RAM in a typical rig. Mining rigs require a minimal amount of memory. You might even get away with 2 gigs on a Linux-based machine. Mining rigs memory should be just enough to run the chosen os smoothly during setup.
2)Each riser might require a watt or so overhead in addition to supplying power to the card. It best not to power more than two risers from a single string of cables. SATA only provides 55 watts and it is possible for a card to draw up to 75 watts, so Molex and PCIE power is usually better than SATA for power to risers.
3) My most stable rig has 6 gtx 1050 ti's on an Onda D1800. It is a board that supports six cards, and it doesn't need risers. My miners with risers are less stable.
4) It sounds like you are describing the BIOS mod to RX-470, 480, 570, 580. I think what you are stating is true for the 4 GB models.