[General question about shares]

I hope this is the right section to ask a rather general question.

My experience is that stock orders are virtually always fulfilled immediately, provided trade is on and the limit price condition is met.

Now, why are shares always available? Surely, the number of shares in a company doesn't fluctuate like the market capitalisation of the company plus those shares are already owned by someone. Since me buying a certain number of shares would require someone else to sell the same number, I don't understand the underlying mechanism, as the number of available shares is limited.  

I'm clearly missing something fundamental. Can anyone switch the light on please? 

Cheers, Frank

[Not suitable for European retail clients. Please call our dealing number to discuss.]

I wanted to buy Airbus shares and had the same error message. If you say you don't offer the Xetra stock exchange for share dealing, how come you do offer Infineon, Covestro and Merck KGAA shares (to name just a few), which are all listed on the Xetra stock exchange?