Assuming all remains linear
Total gain will be X + t dB
Ouput power will be Y + t dBm
Assuming amplifier output is either linear or saturated
If Y+t > U then Output = U, and gain will be U - Y + X
Snip <I mean for example my SSPA max. input is 10dBm
So Does my transmitter output have to be less then 10dBm >
Yes. You could damage the input stage, or at least overdrive one of the stages causing a distorted output.
Snip < What is the relation between gain,output power(1dBm compression point) in a transmitter or in a SSPA >
At 1dB compression point amplifier gain has reduced by 1dB.
Antenna will have an effective radiated power of Input power(dBm) + gain(dB)
Gain can be relative to dipole or isotropic. gain in each case is ether dBd or dBi, usally isotropic.
This gives antenna output as Effective Isotropic Radiated Power.