Each ML is approximatly equal to a pack of cigarettes. But, it's a bit lower nicotine content, so you might smoke more often and go through it faster. Also, if you drip, or use modified cartridges, like the "Straw cartridge" you'll also go through it faster.
And, do a search for liquid and you'll find the latest news on liquids. But to summarize, the major supplier is in China, so they're being affected by tightened security restrictions on shipping. So it'll be more expensive, harder to find, slower to arrive, and sold in smaller sized bottles until the olympics are over and it all goes back to normal.
All in all, in Europe, Freesmoke is your best bet.