Is there a figure on number of developers should work on same Developer Pro sandbox? I see a lot of performance issue and environment is very unresponsive when multiple developers (~5) have enabled debug logs. It becomes impossible to use developer console. It doesn't respond on clicks, navigation and throws no response from server errors. Is there any best practice to follow here?
Coding on Developer-Pro Sandbox you are asking for trouble in the first place. Developer-Pro sandbox is supposed to be UAT-QA and not your usual development environment.
Having a single sandbox where multiple developer's code is like playing a devils advocate where anyone can overwrite someone else's code and there is no tracking and historical code change trends. It can get ugly even before you think about migrating.
A developer should have their own Sandbox where they work and test their code peacefully. Once they are sure they can commit in Code Versioning System and move to Developer-PRO(QA or UAT).
The problems you mentioned is the reason salesforce provides 100's of sandboxes so that each dev can have at least 1 sandbox and can work/test peacefully without facing intermittent issues from other devs working on their functionality.
You might be interested in looking into Salesforce DX which introduces scratch orgs which have life of 7 days that devs can use to work over the week and get going.