All the tracking numbers have letters and number or they can be just numbers?

Registered Air Mail should be have Tracking number with LETTER and NUMBER.

If the tracking number just have NUMBER it untrackable if it already left china. Ex: Yanwen Post

dear , my package is only 11 number and it should be shipped before 6 days .i still can not track my shipment in china is it normal?

There is tracking number that has only 11 numbers. It is normal. No worry.
Your parcel should not be delivered yet. Generally, it should take a couple of days for the delivery agency to collect the package depending on the parcel's volume. You are advised to wait. Or you can call the agency to know the latest information.