GPUs are often used to mine Bitcoin because they are more effective at mining than CPUs. The number of GPUs you need to mine Bitcoin depends on a few factors, including the Bitcoin mining software you use, the difficulty of the Bitcoin mining pool you join, and the speed of your Internet connection.
If you use a CPU to mine Bitcoin, you will likely need to join a mining pool because solo mining is rarely profitable. The pool will likely have a set fee, which will be deducted from your earnings.
The pool will also take a percentage of your earnings as a fee.
The difficulty of the Bitcoin mining pool you join will also affect how many GPUs you need to mine Bitcoin. A higher difficulty means that it will take longer to earn a share of the block reward, so you will need more GPUs to make up for the lower earnings per GPU.
The speed of your Internet connection also affects how many GPUs you need to mine Bitcoin. A slower connection will mean that you will need more GPUs to make up for the lower hashrate.