About Cayenne library

#define LPP_TEMPERATURE 103
#define LPP_RELATIVE_HUMIDITY 104
#define LPP_ACCELEROMETER 113

what is these 103, 104 and 113 in cayenne library ? what they denote ?, can i assign any number ?

that values are used for cayenne LPP data types https://developers.mydevices.com/cayenne/docs/lora/#lora-cayenne-low-power-payload-data-types

Can I define more sensors by doing some changes in Cayenne LPP library?

those are the only supported data types. if you want to send any other sensor type then just send it for analog sensor type.