Database.convertLead() DML count

As stated on Execution Governors and Limits:

Calls to the following methods count against the number of DML queries issued in a request.

• Approval.process
• Database.convertLead
• Database.emptyRecycleBin
• Database.rollback
• Database.setSavePoint
• delete and Database.delete
• insert and Database.insert
• merge and Database.merge
• undelete and Database.undelete
• update and Database.update
• upsert and Database.upsert
• System.runAs

Every time you call any of them, the system increments the DML counter by 1, regardless of how many records were processed by the statement.

I can't verify this is what happens with Database.convertLead, but you can insert up to 10 different types of SObject in one List<SObject> even without using that method, as astutely noted by @Joca in the comments.

insert new List<SObject>
{
    new Account(...),
    new Contact(...),
    new Opportunity(...)
};
system.assertEquals(1, Limits.getDmlStatements());

The reason is that 1 DML statement can insert multiple records. The system created an Account, an Opportunity and a Contact, all with a single Apex DML statement: Database.convertLead(lc);

The number of records created is irrelevant to the number of statements used.