7:1 for rope rodes (with chain)
5:1 for chain
Thats the what the angle of flukes (etc.) makes with the bottom so that the anchor keeps digging .... instead of 'skipping' along the bottom or pulling out/up.
In other than 'light' conditions any less scope and the flukes, etc. will begin to come UP through the bottom instead of attempting to go deeper.
If you want to anchor with less rode, get an anchor with deeper 'setting' angles (flukes) or 'short shank' the stock with a connection further down/closer to the flukes.