I grow a lot of lilies, and, whilst I'm not saying it's not possible, I've never had any with more than one stem. Maybe there are a few bulbs very close together, or perhaps some of the stems are from the tiny bulblets which form around the base. Either way, you would be best to leave them alone until after the flowers. I'm sorry I can't tell you when that will be as your climate is very different from mine.
Once they flower you'll be able to tell whats what, then, after the foliage has died down, you can dig them up and divide them. However, you only need to divide liliums when they become very crowded, so you might want to just leave them where they are for a few more years.
Liliums like a cool root run, so it's important to keep them well mulched. They also appreciate a bit of liquid fertiliser as they are growing up, and several more feeds as the foliage is dying down. If you want to cut the flowers for the house, leave at least half the stem to nourish the bulb for next year.

Thankyou WB (love the name). I only planted the 12 bulbs in that spot and they are all there and accounted for. I am guessing it is the bulblets around the base. The four stems are all of a large size already though. Do bulblets normally grow the same size as the parent plant?
Thankyou for the rest of your advice, I will follow it to the letter. Have never grown liliums before, I fell in love with them when I was given a pot with one already flowering. Unfortunately when it died I threw it in the bin, so sad I didnt plant it out as it had great sentimental value. Oh well you live and learn.

I'm glad of the info too as the school is having a fundraising bulb drive and I was thinking about getting some oriental lilies as well as some other bulbs.

The only bulbs I've ever really grown are freesias years ago so I thought I'd try something different as I've a few gaps they would look good in.

Sounds like your bulbs have increased in size enough to produce extra stems. The smaller bulbs that form up the stem from the main bulb, will take much longer to reach blooming size.

Here's an interesting page for you.

Here is a link that might be useful: Lilium Propagation

Now that I know you planted the liliums this year, my guess is that they are large healthy bulbs which are already sending out shoots from some of the scales, (the fleshy white bits that make up the bulb). Liliums do this all the time, and some people separate the scales as a way of increasing their plant stocks. I read that it can take up to 10 years for scales to grow into bulbs large enough to produce flowers, and a little less time for bulblets.
The best way to increase your lilium display is to take advantage of the bulk discounts a lot of the mail order companies offer. You usually only have to buy 20 or so of the one type. You might also like to try asiatics, which aren't fragrant like orientals, but come in a much wider range of colours and flower earlier. I'm sure you'd like them.
Happy gardening