Question:
The key here is "entirely" self sufficient, or just "self sufficient".

Mere self sufficiency is that the unit in question will survive ifthe transport/commerce are cut off, if perhaps not thrive.Entirely self sufficient requires that it have within itselfcomplete resource sets to be able to continue on largely asnormal, without significant reduction in capabilities.

For a one-way Mars mission not contemplating children and furthergenerations on Mars (at the outset), mere self sufficiency couldbe as simple as around 25 tons per person worth of supplies andrepair parts. EVA suits and rover parts are the critical resources,as far as I can tell: everything else can be made out of equipmentthat has essentially no, or very low, fatigue or burnout problems.CO2 and H2O adsorbtion from the atmosphere can be with only 2moving parts (one 3-way valve, and one fan/compressor).Reverse osmosis water purification needs pumps, but most ofits mass is in the osmosis filters which can be designed andoperated to last forever, which means that a few spare pump partswill ensure long long life. If you don't trust biological ECLSSfood and oxygen, you can store 3 or 4 man-years of good qualityfood per ton and get oxygen out of either CO2 or H2O with abit of electricity and a few valves.

Answer: You're on an island. You have a small bulldozer, some computers,a house, generator, small garden. You go scuba diving and motorboatingaround the island. You rely on a monthly supply ship to bring youfuel and food and repair parts.

I cut off the supply ship permanently. You go "uh", and expand yourgarden to grow more food, using the last of your bulldozer/car/generatorfuel in the dozer. So the lights, vehicles, computers are now down for good.Now you're fishing with homemade nets and lines, not scuba diving,because you can't compress air for diving without the generator running.You are able to make enough food on an ongoing basis to live,but you're not nearly living at your prior lifestyle. You are selfsufficient.

Someone else on a similar island. They have a generator, but also havea bank of solar cell panels for electricity. They own a sailboat insteadof a motorboat. They own a machine shop, electrically powered, and havegood stocks of metal to work with. Cut off their link. Ok, they also usethe rest of their fuel to run the bulldozer to expand out the garden togrow more food. But they still have electricity from the solar panels,so they still have lights and computers and a compressor for scuba air.The sailboat still works fine, so they're still sailing around theisland and have a more effective fishing platform than fishing from land.A couple of years later, they've taken the former internal combustionengine bulldozer and converted it into a crude single expansion steambulldozer, and are using it to slowly expand the garden even further.They're "entirely self sufficient", they haven't lost any capabilities.However, if they have 10 kids, they can't make another bulldozer,can't make more solar panels, don't have more computers, can't minemore iron etc., so the next generation isn't going to have the samelevel of living conditions.