The description of Free Actions on page 163 of the core rule book for 5th Edition covers your question, I believe. Essentially, this is a situational determination based on the description from the player, the sequence of events, and the GM's ruling. It's not set up in a comparative relationship like 2 Simple Actions = 1 Complex Action. Free Actions are an aspect of activity in an Action Phase that cover all the little things which could be occurring at the same time as the meatier actions.
In the question, a sequence of events is provided as an example. Where that sequence runs into some trouble is that it is a linear series of discrete actions. Free Actions are minor things (dropping something, saying a word) which require little to no attention to carry out and can be completed while the character is performing their Simple Actions or Complex Action.
Using a Simple Action to Take Cover and adding a Free Action after that to Drop Prone, but pausing to communicate in word and gesture before they Take Cover and somewhere along the way dropping what is in their hands is attempting to compress a lot of time into the Phase and focuses on one thing happening after the other. If the player can describe how it works, and the GM is fine with it, that is fine. The intention of the rules seems to be to not control and limit every aspect of character action, while also not leaving a large hole for abuse. As an example of this, if the character wants to run instead of walk, it burns a free action. Basically, characters have 1 free action. Only allowing one Free Action would mean to some reading the rules that you cannot run and drop something at the same time, hence the freedom to expand the amount of free actions to fit the shared view of the scene. What I think is a key point of all of this is that Free Actions are occurring as a part or as a compliment to the Simple or Complex Actions which the character is making in the scene, and occur more or less at the same time.
Were I to be asked to make a decision in this case because the outcome of the scene hinged on whether or not a character could do all of these things in one Action Phase, I would lean toward suggesting that the sequence could more easily be resolved by having the character dive for cover while dropping the object and shouting. This covers their two Simple Actions (Moving to Cover and Taking Cover), and does not require a lot of Free Actions: dropping an object while running, speaking a short phrase while running and taking cover. The two simple actions may occur more or less sequentially, and the Free Action(s) occur during one or both.
It's not so much trying to squeeze as much into one phase as possible, but trying to break down what is actually happening within a (3 second or so) frame of time.