I am using STM32G0. I want to use 8 channels for PWM . While choosing RTOS it suggested to use one channel as time base. If i do that i have left with only 7 channels.

you meant you use 8 timers for pwm and FreeRTOS is asking to use tim17 as systick source so now youre left with only 7 timers?

Each timer has different channels for pwm generation inside.

TIM1 has 6 pwm channels available

TIM3 has 4.

Those add up to 10 pwm channels you can use and you have 3 spare timers to use for whatever you need.