WACopyMate is a dead simple Firefox add-on to copy contact numbers from chats on WhatsApp Web in 1 click. With this, you will not have to manually copy numbers, remove spaces, and remove parenthesis. Just tap on the ad-don's icon in the toolbar to copy number of the contact in currently opened chat.
For unsaved contacts, there is no need to do anything extra. Just open the chat and click on the add-ons icon to copy the phone number to clipboard.
To copy number of a saved contact, you need to click on contact's profile once and when the contact info sidebar appears, click ad-don's icon to copy the phone number to clipboard/
By default, it will skip the country code. But if you want country code to be included in the number then, go to preferences of the add-on and then check "Include Country Code" checkbox.
Icon credits: Free Duplicate Icon by Mohit Gandhi
Enjoy.