The luggage allowance with your ticket is for all your flights together. Any luggage you check as you depart A has to be within your allowance. Once you leave B, that luggage will still be loaded onto the plane and will count just the same.
Conceivably, you can also pay for additional luggage. People usually do it on departure from the first stop but you might be allowed to buy more allowance at B and have it checked in. That might require you to get out of the terminal area to get to where baggage is dropped off, it depends on the airport's layout.
The exact policies depends on the airline but usually you have a weight allowance and number items you are allowed to check. Most airlines specify weight per item but LATAM specifies a total weight for all items. So if you have an allowance of 46kg and you checked in 24kg, you could add another item for 22kg. The actual rules are rather complicated and depend on the route. This is the first time I found rules that work this way but I supposed there might be other airlines doing similar things.