The short answer
Water with a TDS of up to 500 ppm meets the acceptable limit of the Indian drinking water standard, IS 10500:2012. Water up to 2000 ppm is permitted only where no better source exists. There is no minimum. Within that range, most people find water somewhere in the low hundreds tastes best, and that is the figure to aim for at the tap if you can control it.
That is the whole answer. The rest of this page explains why the question gets asked four different ways, what each version is really after, and why the "best" number is not the same for a municipal tap, a borewell and an RO purifier.
Best, ideal, safe, minimum: four words, one question
Someone asking for the safe TDS wants to know they are not harming their family. The standard answers that directly: up to 500 acceptable, up to 2000 tolerated. Someone asking for the best or ideal TDS usually already has safe water and wants to know whether their reading is good or merely acceptable. That is a taste question, and taste sits well below the legal ceiling. Someone asking for the minimum TDS has usually just read that RO water is "dead" and wants to know how low is too low.
All four are asking where the good water is on the same scale. Splitting them into separate answers only invites four slightly different numbers, which is how the confusion started. The full scale, with every band and what it calls for, is on the TDS reference page. This page is the part of it that deals with the good end.
What BIS actually says, and does not
The Bureau of Indian Standards publishes IS 10500. For TDS it gives an acceptable limit of 500 mg/L and a permissible limit of 2000 mg/L in the absence of an alternate source. It gives no lower limit, because the standard exists to keep harmful things out of water, and low mineral content is not, in the standard's view, a harmful thing. A milligram per litre is a part per million for water, so those figures are 500 and 2000 on your meter.
Two things the standard does not say are worth spelling out. It does not say that 500 is a safety cliff; 520 ppm water is not dangerous where 480 ppm water was fine. And it does not say that TDS alone makes water safe. The same standard carries limits for fluoride, nitrate, heavy metals and bacteria that a TDS meter cannot see. Water at 200 ppm with too much fluoride fails the standard; water at 900 ppm with none of the listed contaminants passes the permissible limit. If that surprises you, the guide on what TDS does not tell you is the one to read next.
Why the best figure depends on where your water comes from
Municipal supply. Treated surface water in most Indian cities reads somewhere in the low hundreds. If that is you and a lab report is clean, the water you have is already close to the best figure, and the useful purchase is disinfection rather than RO.
Borewell. Groundwater picks up mineral from the rock it sits in, so readings above 500 are common and readings above 1000 are not rare. Here "best" is not a number you can choose at the source; it is the number you set on the purifier. What matters more is what rides along with the minerals, which is why borewell owners should test for fluoride and nitrate once. The borewell TDS guide covers that.
RO output. A membrane takes almost everything out, and the water leaving it can read in the tens. That is safe by the standard, but many people find it flat. Purifiers with a TDS controller let you blend a little unfiltered-by-RO water back in to bring the figure up. On a clean municipal feed that is exactly what the controller is for. On a feed with fluoride or nitrate above the limit it reintroduces the problem, and a mineral cartridge that adds minerals after the membrane is the safer route to a better taste.
Why very low TDS is not better
It is tempting to treat TDS like a score and chase the lowest reading. Do not. A very low reading means the water carries almost no dissolved mineral, and the certain consequence is taste: water at 20 ppm tastes flat to most people, and some find it faintly sour. The health consequences of drinking very low mineral water long term are argued about and not settled; what is not argued about is that most of the calcium and magnesium in a normal diet comes from food, not water. So very low TDS is not dangerous, but it is not an achievement either. It is simply RO doing more than the water needed.
The practical target is water inside the acceptable limit that tastes good to you. For most households that means somewhere in the low hundreds. The low TDS guide goes into what the membrane strips, what the flat taste actually is, and how a controller and a mineral cartridge differ in fixing it.
Setting the figure on a purifier
If your purifier has a TDS controller, set it once, in the direction of taste, and check the output with a meter afterwards. In the LUXGEN range the Royale Copper has an adjustable controller and the Aura has a controller as well; the Curve does not, so its output is whatever the membrane gives. On any of them, the number to keep an eye on is not the output alone but the gap between feed and output, because that gap is the membrane's health. The RO water TDS guide explains how to read it.
Questions
What is the minimum TDS for drinking water?
IS 10500:2012 sets no minimum. It gives an acceptable limit of 500 mg/L and a permissible limit of 2000 mg/L, and nothing at the low end. Very low TDS water is safe by the standard; the practical objection to it is flat taste, not health.
Is 100 ppm TDS safe to drink?
Yes. It is well inside the 500 mg/L acceptable limit. Whether it tastes good depends on the person; many find water in the low hundreds pleasant and water below about 50 flat. If it is RO output and you want more taste, raise the TDS controller a little.
Is 300 ppm TDS good or bad?
Good. It is inside the acceptable limit and typical of treated municipal water. If it comes from a borewell, expect it to rise in summer and treat 300 as a winter figure rather than a fixed one.