As Friday's iPhone 5s and iPhone 5c release approaches, analysts and Apple watchers have been predicting how many of the new iPhones Apple will sell over the launch weekend, and predictions range from 5-8 million units. However, there is some concern about possible iPhone 5s constraints and low pre-orders of the iPhone 5c.

Piper Jaffray analyst Gene Munster has said that Apple is likely to sell out of iPhone 5s units this weekend, and believes that the company will sell 3 million iPhone 5cs over the three-day period.

See: iPhone 5c pricing and tariffs

And: iPhone 5s pricing and tariffs

Last year, Apple sold a record-breaking 5 million units in the first three days of availability. Those sales were made up of around 3 million in-store sales and 2 million pre-orders.

"Given last year's in-store sales numbers, we are comfortable that Apple could sell 2.5 million iPhone 5s units this launch weekend," Munster told investors in a new report this week. "Our estimate for the iPhone 5c is 3 million units sold during the launch weekend including 1 million online pre-orders and 2 million in stores."

See: Best cases for iPhone 5c

Earlier this week, reports suggested that the inventory of iPhone 5s units is "grotesquely low," and Munster believes this to be true.Plus, a new report from the Guardian says network operators in the UK are reporting that pre-orders of the iPhone 5c are 60 to 70 per cent below expectations, and that it has been "a big disappointment."

If the number of iPhone 5s units really is extremely constrained, it could mean lower sales than expected, but not due to lack of demand. Customers have been unable to pre-order the device, which some people believe is a sign that iPhone 5s units are limited.

Munster says that, if Apple doesn't announce launch weekend sales, it could be a sign that sales were below 5 million. Munster advises that, due to possible constraints, it might be better to focus on how fast the iPhone 5s sells out in order to determine the demand for the device.

See: The history of the iPhone

Meanwhile, KGI Securities analyst Ming-Chi Kuo has sent a note to investors predicting sales of 6 to 8 million iPhone 5s and iPhone 5c units, and that Apple will announced the overall sales of the two devices after the launch weekend rather than disclosing numbers for each.

Kuo believes that, due to supply constraints, the iPhone 5s will account for 35 to 45 per cent of the sales, with the iPhone 5c accounting for the remaining 55 to 65 per cent.

See: Best cases for iPhone 5s, iPhone 5

As for the price of the iPhone 5c, Kuo says that he sees the new model as an extension of the life of the iPhone 5, rather than a low-cost iPhone many consumers were expecting. He says that, if the iPhone 5c had been priced too low, it would have risked cannibalisation of the iPhone 5s sales.

Kuo notes that Apple could decide to reduce the price of the iPhone 5c at a later date, perhaps when it unveils the iPhone 6.

Both new iPhones will go on sale at 8am local time on Friday at Apple Stores across the globe.

Apple fans have been queuing outside stores in New York for almost two weeks already, while fans in London arrived at Regent Street's store on Monday.

See also:

15 October Apple event: iPad 5, iPad mini 2, Haswell-powered iMac, Mac mini, MacBook Pro coming?