Is it considered rude to order more than one item at a counter in a store if other people are waiting in line?

Think about how much time you'd waste by ordering one item and then going back to the end of the line to order something else.  You waited in line just like everyone else.

One order often consists of more than one item. So no. 

You wait your turn then order everything you want to buy. 

It is acceptable to order multiple items. To be considerate, decide what you will order before your turn comes. If it is almost your turn, and you know you (or someone ordering with you) is not ready to decide, it is nice to offer to let the next person behind you go in front of you so you can finish deciding while they take their turn. 

i usually just get as much as i want and if i see someone behind me with only one or two items i might ask them if they want to go ahead of me

Not only "acceptable" but totally normal. It would be extremely odd if you were to order one item then rejoin the queue.

Of course, if you want to order a LOT - I mean, like two dozen loaves and four cakes and thirty scones and twenty doughnuts - you should call ahead to place your order, as otherwise they might not have all that available when you got to the counter.

But in general people queue in the knowledge that the people in front of them may well be ordering several items.

it's not rude to order all at once.  It is rude if you haven't decided yet, and take your time figuring it out when it's your turn.  Figure it out while you wait in line.

Not at all.  Everyone waits in line to be waited upon and then gets all that they intend to purchase.  

IMHO the rude person is the one who doesn't know what they want when it is their turn and wastes time figuring it out or gets chatty with the person behind the counter while figuring it out.  That person I mentally smack on the backside of the head.

when i go to the dollar store and want to get a bunch of stuff i just get in line and if i see someone with one or two stuff i just tell them they can go first

I do not consider it rude to order multiple items at a bakery or deli counter.  

What I do consider rude is when someone has waited in line but still don't know what they want when they get to the counter.  It is the people who take FOREVER to decide what 10 things they want that is annoying and rude.

It would be crazy for people to stand in line a separate time for every item they wanted and pay for each item individually.  It would take far, far longer to service everyone.