It is possible to use one or more spare parts on an activity. To do this, simply check the "add spare parts" option when creating the activity, then chose the spare part you want to use, and remember to enter how many you need to take out of the stock.
Then, if you want to use another spare part, just click on the "+" icon and repeat the same operation as mentioned above.
The number of spare parts that can be released on an activity is not limited.