When managing multiple accounts, the ideal number of accounts per account is one, but you will agree with me that this could quickly add up to your cost. For this reason, you can manage few accounts using the same IP address, but the actual number depends largely on the social media platform and quality of the proxy.
You can manage 2-3 Instagram or Facebook accounts with one IP. For Twitter, you can manage up to 10 while the number is put around 5 for Pinterest. One thing you need to know is that you can use the same Facebook IP for Instagram and Twitter since the services are different.