For me I always have most of the paper
charts, if not all, for any particular area, but for those who are relying solely on chart plotters I think 3 units is a minimum when you are off long distance cruising. It might be one chart plotter, one PC with
charts, and one
iPhone with charts, but then you have redundancy when one or more units fail. I see way too many
boats that have no charts at all other than the main chart plotter, which leaves you with nothing when it goes out. Like I said before, I had two different PCs die on me when down in the SW
Caribbean and there was nowhere within a couple of hundred miles to get anything fixed or sent in. So that wiped out all electronic charting. Had to go with just the paper charts a basic
GPS giving lat/long and hand plotting for quite awhile before I could get the
computers fixed. Then when we did get to
Cartagena it wasn't all that easy to find the computer stuff we needed. Luckily, my father flew in with a suitcase full of stuff for the
boat. While there I noted many other cruisers in similar circumstances, often working on the third backup for critical
gear. Stuff just breaks, and you need backups. That's one thing nice about paper--it doesn't break.