To encourage us to save, press releases sent to financial journalists at this time of year are full of top tips on how to become an ‘ISA millionaire’, while newspapers are full of success stories.

It might sound great but a quick scan through the expert advice reveals one vital requirement in making it into the millionaire club: you need loads of cash to begin with.

Last year AJ Bell claimed it had “never been easier to become an ISA millionaire”. The investment platform then proceeded to advise investors to invest their whole £20,000 limit into an ISA each year, achieve 7% returns… and then, hey presto, they’d be a millionaire in 20 years’ time.

InvestingReviews.co.uk made similar claims this year. It says investors starting from scratch today could expect to reach millionaires’ row in around 22 years by maxing out their £20,000 annual allowance, assuming a compounded 7% annual return.

See? It’s simple. You just need to stash away that spare £20,000 that you have. Every single year. And don’t spend any of it. On top of that you’ll need to invest in all the right things.

According to the Office for National Statistics (ONS), the average full-time UK salary in 2021 was £31,285, down 0.6% from the previous year due to the impact of coronavirus. This equates to a take-home pay of £24,936.

So, assuming you could live on £4,936 a year, I suppose technically you could save £20,000 a year. But given that most people spend more than that on their mortgage or rent, never mind bills and food, it would be ambitious at best.

A series of Freedom of Information requests by Investingreviews.co.uk revealed that there are actually only 2,000 ISA millionaires in the country – so not as many as the headlines might suggest. These millionaires have average holdings of £1,412,000. Among these are 60 so-called ‘ISA beasts’ sitting on pots of £3m plus – with the average among those standing at an eye-watering £6.2m.

But even the experts accept that building up a £6m ISA pot is no mean feat. AJ Bell worked out that someone who put the full allowance in their ISA each year since their launch in 1999 and who earnt a steady 5% return a year would have built up a pot of just over £441,000 – not even close to being an ISA millionaire, let along amassing £6m.

Even someone who had started saving in ISA’s forerunner – PEPs – and had saved the full limit each year since PEPs launched in 1987, getting that same 5% return a year, would only have built up about £708,000 in their pot.

So how have the people with £6m in their ISA pots done it? AJ Bell worked out that if they started investing when ISAs launched in 1999 and put the full allowance in, they’d have to have earnt 25% returns a year on average every year. Even including PEPs they’d still have to have achieved returns of 15.5% every year to get to a pot of £6.2m today. Where can you achieve 15% or 20% on your investments? Drop me a line if you know, please.

Given PEPs were only launched in 1987 with just a £2,400 allowance, the data released by HMRC indicates a small number of investors have benefited from supercharged returns over the years. AJ Bell explains: “These investors probably made a few very savvy bets that rose dramatically in value and a few more stable returners.”

Every year during ‘ISA season’ we see the same advice banded around about becoming a millionaire – but it’s rarely relevant to the majority of savers earning the average salary without rich parents or an inheritance to bump up their savings. Or those just achieving average investment returns.

Of course, saving in an ISA is a no-brainer. The returns are tax-free and investing in the stock market over the long-term will generally reap rewards. Regular saving or ‘pound cost averaging’ can smooth out the ups and downs of the market.

But only a small percentage of people will become millionaires this way – so don’t be too disappointed when it’s not you.