How to calculate capital gains on multiple trades of the same stock?

Adjusted cost base
In some cases, special rules may apply that will allow you to consider the cost of a property to be an amount other than its actual cost. This section explains these rules.

Identical properties
Properties of a group are considered to be identical if each property in the group is the same as all the others. The most common examples of identical properties are shares of the same class of the capital stock of a corporation or units of a mutual fund trust.

You may buy and sell several identical properties at different prices over a period of time. If you do this, you have to calculate the average cost of each property in the group at the time of each purchase to determine your adjusted cost base (ACB) (dispositions of identical properties do not affect the ACB).

The average cost is calculated by dividing the total cost of identical properties purchased (this is usually the cost of the property plus any expenses involved in acquiring it) by the total number of identical properties owned.

Any amount reported in box 42, "Amount resulting in cost base adjustment," of the T3 slip represents a change in the capital balance of the mutual fund trust identified on the slip. This amount is used when calculating the ACB reported on Schedule 3, Capital Gains or (Losses) in 2020 for the property in the year of disposition.

source: https://www.canada.ca/en/revenue-agency/services/forms-publications/publications/t4037/capital-gains.html#P1185_82580