The number is determined by the purchase price and is automatically calculated by PP. You have to compare this with your billing statement and possibly correct it.
The most important thing for saving plans is the rate you invest per saving (and fees) and the exact amount of shares you buy.
In PP you can automate the saving plan bookings with date and invest per saving. Since PP calculates the amount of shares from an online price you have to edit every automated bookings to fullfill my first sentence.
Note: there is a quite good online translation called deepl.com. There you can phrase in French and get quite good translations in e.g. German
In addition to this, when you import your billing statement from the particular buy, PP will auto. adjust the number of purchased shares and it’s price.
The importer will show you an exclamation mark during import, saying the values will be adjusted.