Trading is not conducted individually, but in lots: an order is placed for a whole number of lots. The lot size is set by the exchange for each instrument and can range from a single security to dozens or hundreds.
Standardization is intended to simplify order matching and reduce processing costs for small transactions. A side effect is the entry barrier: if a lot includes a large number of securities, the minimum transaction amount becomes significant for a retail investor.
Lot size affects the ability to build a portfolio with the desired structure. With large lots, it becomes impossible to distribute a small amount across multiple instruments, limiting diversification mechanically rather than by investor choice.
When planning a purchase, it is advisable to clarify the lot size in advance and calculate the actual minimum amount, as well as check whether it has changed after corporate actions—stock splits are usually accompanied by a revision of trading parameters.