Actually I researched this because I didn't understand the question and was applying for a job that required an answer. I have a very nice well-paying client who assigns me milestones which to us is the same thing as projects. Each one is competed and I get paid for it. I don't do a draft or add images in various stages of completion, each milestone is just a simple project. I still don't think I understand the purpose of milestones other than to set a point in a project where the client pays you. I guess I just put "1" every time?
Actually, it depends on many factors like project complexity, contact length, contract price itself, etc.
Anyway, whatever the number of milestones are in a particular contract they are spelled out in a very details in the proposal - meaning is that client at the very begining of the contract knows exactly what is he/she going to pay, how much and when.
Sometimes it happens during the contact executation that we together with client adjust milestones based on actaul work.
Simple as that ...
Most of my projects require reading 100's of pages of specification before work can even start...
In one project I had to spend 1 week chatting with the client to transform his vaguely worded RFQ to a proper specification
Unfortunate split into multiple milestones does not work. We are highly specialized into logo, label and packaging design. Most of our projects are about two and more labels and packaging designs.
Typically clients logically split the project into:
Milestone 1: First label. (M#1)
Milestone 2: Second label. (M#2)
Milestone 3: Third label. (M#3)
After we have a label design ready (M#1 for example), we send print files to the client. Client re-send print files to third-side to get final check and approval (manufacturer, attorney or print service for example). This takes unpredictable time (typically two or more days) and sometimes we should make small revisions on the labels or packaging texts/copy (to fit into requirements). While we are waiting for final approval (M#1), often clients ask from us to start working on the next label (M#2). Upwork system rules does not allow to do that, because we do not have activated milestone (M#2). To activate the milestone (M#2) client should approve the previous one (M#1) (but we should wait final approval from third side). At this point, we could pause the work or continue to next one (M#2) in our risk, or client should approve the active milestone (M#1) and activate next one (M#2).
So the question is, why upwork does not allow to have two or more active milestones at the same time?