The fifth challenger in the 36th America's Cup in Auckland is thought to be US21 and the Long Beach Yacht Club – a second American challenge for an event looking as if it will be much beefier than many thought, with six challenges a possibility and a slim chance of seven.

Make no mistake – though the new entries received recently by Emirates Team New Zealand are "late" challenges for the 2021 event, the conditions of entry (including US$4m of fees and performance bonds by the end of December) and the need to catch up on a difficult boat design mean these are no lightweight, funsy, let's-go-for-the-hell-of-it challenges.

US$4m up front is, even for billionaires, not chump change and represents a pretty firm commitment – though it may be some teams will seek an instalment plan to help with the financial burden.

Yesterday's confirmed announcement of the Malta Altusa challenge may also be shrugged off by some who think the tiny Mediterranean tax haven is likely only to come away with the title of the smallest country ever to compete for the Cup (previously held by New Zealand, of course).

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But not only does this challenge have the money, there are strong signals they are putting together a good sailing team involving some from Artemis – the 2013 and 2017 challenger not competing this time. Principal among them is said to be Iain Percy, the Olympic gold medallist and highly experienced and respected America's Cup sailor.

Malta might seem an odd contender but sprang out of the long-anticipated second Italian challenge. One version of the evolution of this challenge says at least one Italian yacht club wanted a large donation to be the challenging club – so the organisers hitched their wagon to the Royal Malta Yacht Club instead through patron Pasquale Cataldi, head of Altus, who lives in Malta.

US21, now challenging through the Long Beach Yacht Club from California, are said to be the other syndicate accepted by ETNZ; they are headed by hot US match racer Taylor Canfield.

So that's five challengers: Italy's Luna Rossa (the challenger of record, COR), INEOS Team UK (Sir Ben Ainslie's mob this time round), American Magic and the New York Yacht Club plus Malta Altus (Altus being the real estate company behind the challenge).

The six other conditional late entries still in front of ETNZ need to have certain requests granted before they can join the hunt – and those conditions also must be ratified by Luna Rossa as COR.

So it will be a little while yet – maybe the new year or after – before we know the identity of confirmed challengers for the 36th America's Cup. However, the volume of bar-room gossip/sailor speculation is rising, pointing to some possibly left-field entries.

Like a joint China-Japan entry, yet another Italian entry or a Norwegian challenge; there are persistent rumours of a Dutch syndicate too.

Of those, perhaps the third Italian and the China-Japan efforts look the most likely though they may yet stumble on the money and/or other fronts.

Conditional entries require changes to the protocol that Luna Rossa must agree with – possibly not an automatic occurrence; any challenger would want to weigh up the effect on its own challenge of allowing more in.

Perhaps the third Italian syndicate, said to be near Genoa in northern Italy, is the most likely; China and Japan do not have historically close ties and would most likely be funded by governments. Some years ago preliminary attempts were made to involve Singapore in an America's Cup challenge – but while Asian governments are keen on regattas and entering the global yachting circus, they generally baulk at funding America's Cup challenges.

It may be that China and Japan are satisfied this time round with a World Series regatta (where points are gained for the challenger series in Auckland) ahead of the Cup, if that works out. The first of those preliminary races is due to take place in Sardinia next October.

However, if they can find the corporate backing, the Sino-Japanese team could become a reality or they could still form single-country challenges with the Chinese counted as more likely.

The big conclusion from all this, however, is that the 2021 Auckland regatta has grown in terms of the confidence shown in the event by other teams – given the high cost of entry and developing a new boat, the foiling monohull AC75.

Five challengers equal the number at Bermuda (although one of those 2017 challengers was Team Softbank Japan, propped up by Oracle Team USA giving them a design package and a helping hand for that regatta) and is way ahead of the 2013 Cup in San Francisco when 10 syndicates originally challenged but most dropped out, leaving only two (Luna Rossa and Artemis) to race against ETNZ.

Team NZ, knowing such things can happen in the Cup, have played their cards close to their chests so far – but if six teams sign up, that will be the highest number since the 11 challengers at Valencia in 2007 when Alinghi still held the America's Cup.

Six will also trigger the Government's commitment to build an extra team base on Hobson wharf (there are five on Wynyard Point). If they get seven, a base will have to be found for that challenger somewhere else in Auckland.