The TextInputLayout has a character counter for an EditText defined within it.
The counter will be rendered below the EditText.
Just use the setCounterEnabled() and setCounterMaxLength methods:
TextInputLayout til = (TextInputLayout) findViewById(R.id.username);
til.setCounterEnabled(true);
til.setCounterMaxLength(15);
or the app:counterEnabled and app:counterMaxLength attributes in the xml.
<android.support.design.widget.TextInputLayout
app:counterEnabled="true"
app:counterMaxLength="15">
<EditText/>
</android.support.design.widget.TextInputLayout>