You have to add the variable reference using Dynamic content section.
Follow steps:
Go to flow action > open Dynamic content pane
Go to Expression tab
add this formula: addDays(utcNow(),) and move cursor after the comma (,)

Click on Dynamic content option again and you will be able to select previous action outputs from there, like:

Select variable you want to add from dynamic content section and Click OK.
Expression should look like:
addDays(utcNow(),triggerOutputs()?['body/NumberCol'])
Where, NumberCol is the internal name of your SharePoint list column.
triggerOutputs()?['body/NumberCol'] - this part in expression will change as per your action name & action response structure.
For example: If you are using a Power automate flow variable, it will change to:
addDays(utcNow(),variables('Czasookres'))