If you have builders hired, you don’t have to worry about the Not Enough Builders warning. The dialog window indicates the status of the construction work in real-time. That means that your constructors are on the way from the builders’ room, or from other construction work. 
 
Once they get there, the warning will change into the specific number of builders working on it, and you could also see the remaining time for this specific building. The more builders you have, the quicker they finish the construction. 
 
If builders are working in one building, they will automatically go to other construction works after finishing, so you can open simultaneous building projects.