There are around 6000 DTC buses with following breakup:
DTC-5000
DIMTS-1157
METRO FEEDER-117 (small buses who take you to metro stations.)

The DTC buses are GPS enabled and use CNG as fuel rather than any petroleum bases fuel. The buses go throughout the inner and outer ring road as well as the express highways.

There are 3 colored buses in Delhi viz.
Red: The AC buses with 25% higher fare than ordinary green line buses. They are available on almost all major routes of NCR.
Green: These are the ordinary buses with out AC so they are more commonly seen in Delhi and the same are available throughout the NCR.
Orange:

The following numbers and offices are for your help in case you encounter any problem while using the transport method:
Office: Public Relations Officer,
Transport Department,
5/9 Under Hill Road ,
Delhi 110054.
Phone No. 23994223
Email : protpt@hub.nic.in
jdadmntpt@hub.nic.in

Helpline: 42-400-400

Online License Help Line
Centre No. 9311900800

Complaints Helpline: 155345

Complaint regarding over-charging/refusal/misbehaviour by auto-rickshaw/taxi/local buses :
Complaint cell Tel.No. 23989091