How do I set that the signature can be placed after a certain number of posts and insert a warning that reminds the rules when inserting the signature?
You would disallow signatures in Registered and then create a new group, say Permission: Signatures. Then, create a User group promotion to add them to the new group, that has permissions for whatever signature settings you allow, after X posts are made.
Permissions for Registered:
Permissions for Permission: Signatures:
User group promotion: admin.php?user-group-promotions/
Create a new one.
Add user to user groups: Permission: Signatures
Apply this promotion while tab...
As far as reminding them about the rules of a signature, you could firstly create a Rules thread outlining it. Further, you can also reinforce those rules by enabling the notice system and displaying a notice on Content template is: account_signature set under the Page criteria tab for the notice that you create.
It would look something like this, shown only on the edit signature page.