This really depends on where and under what conditions one plans to sail. On Lake Ontario a GPS and copy of Richardson's is fine. We sailed with much less for years, a paper chart, a compass, and a boom box and thought nothing of it.
We outfitted Second Star with a full suite of B&G electronics. At the helm we have a 9" Zeus2, 4g Radar, Triton2 instruments, and a below decks AP. At the Nav station we are adding a Simrad GO7 XSR. We also use iPads that can display and control the Zeus or GO 7. There are several more GPS's onboard in phones, old handheld GPSs, and VHFs.
We installed all these in preparation for an extended cruise from Lake Ontario to the Bahamas via the St Lawrence River. The Simrad is new addition. We wanted to tank level sensors and to have instrument data available below decks, turned out the cheapest way to do that was to purchase a small CP.