Hi @ Dany,
Do you want the "Get items" action would return all items when the "Top Count" field is left blank?
Microsoft flow 'Get Items" action support to returns 100 items when "Top Count" box is left blank currently,
if you want the "Get items" action would return all items when the "Top Count" field is left blank, please submit an idea to Flow Ideas Forum:
https://powerusers.microsoft.com/t5/Flow-Ideas/idb-p/FlowIdeas
In addition,if you would like to set the "Top Count" like "9999999",there is a 5000 items limit of sharepoint items, so the "Get items" action would return "Top Count" 5000 items currently,I afraid that there is no way to achieve your needs in Microsoft Flow currently.
Regards,
Alice Zhang