Others can explain this better than I but there are some links in the Beginner's Sticky that will explain it too.
I'll have a bash myself but as I say I'm not the most knowledgeable.
Basically you can take as many or as few as you like. You have to organise them into groups of no more than ten. Ten is preferable as these soldiers provide and share an order pool. Most games are played at 150-300pts and most lists consist of as close to ten models as possible and they are placed in one group. This allows you to disperse orders however you feel best fits your situation.
Nothing stops you from taking 50 Glawegians though for instance. Or a couple of TAGs.
You should also take into account whether troops are regular or irregular. Irregular troops do not share their orders with anyone else. In that way if you only took irregular orders you would only have one order per miniature and one extra for your lieutenant.
In Infinity we say, "It's not your list, it's you." But that doesn't mean a bad list will hurt you, it just means that there aren't any overpowered lists. Not really anyway. If you think the list seems overpowered in some way then you'll find there is a weakness to be exploited somewhere else.
For these reasons most lists are well balanced with a few different types of soldier and a wide range of expenses. It's also important to remember that a more expensive model won't always be your most important. A cheap hidden lieutenant can help you snatch victory from the jaws of defeat. A doctor or an engineer can turn the tide of battle in one turn. And a few cheap hackers working together can take down the biggest, badest piece of armour you've ever seen.
Hope that helps.