I work in a business of more thant 15,000 persons. Normaly we are running local NFS for our offices but we are looking to merge everything in one single Sharepoint and replace personal drive and shared file and folders with Sharepoint. Is it a good idea? What are the negatives and positives of that architecture. How many Sharepoint specialist will we need to maintain that kind of architecture?
This is a very objective question. Personally I would look at O365 and utilize OneDrive for Business (1TB per user, syncs locally).
If you need to deploy on prem you need to start digging into SharePoint requirements. SharePoint stores all data in SQL so when you size it you need to size it for SQL DB data. you also need to determine what size OneDrive for Business site will be for each user and distribute them between databases. If you want search to work - you also have to consider how many and the total size of documents you will be indexing to plan that out. As for how many people it will take - it depends on the people.
All in all there is not a good way to answer your question here. I recommend you find someone very experienced or contract with one. If you have an enterprise agreement you can use Microsoft Consulting Services to help you design it as well.