Volume is measured in lots
You do not buy "some euros". You buy a lot, a fixed block of the base currency. On forex pairs a standard lot is 100,000 units — that is the contract size, and you can find it for every instrument in the symbol specifications.
| Name | Volume | Units of base currency |
|---|---|---|
| Standard lot | 1.00 | 100,000 |
| Mini lot | 0.10 | 10,000 |
| Micro lot | 0.01 | 1,000 |
MT5 lets you type any volume down to 0.01, so 0.35 lots is 35,000 units.
Why it matters
Volume decides two things at once: how much each pip is worth, and how much margin the position needs. Doubling the volume doubles both. A beginner who "just tries 1 lot" on a 500-dollar account has, without noticing, put on a position where a 50-pip move is the whole account.
Not every instrument is 100,000
Gold (XAUUSD) is typically 100 ounces per lot; indices and crypto CFDs each have their own contract size. The formula is the same everywhere; the number is not. Check the specification before you trade something new.
Summary
A lot is a fixed block of the base currency; 1.00 is 100,000 units on forex pairs. Volume sets both pip value and margin, so it is the risk decision in every trade.