X character counter
280, unless the account pays
X Premium raises the ceiling to 25,000 characters. Nothing in the API says whether an account has it, so any tool that counts for you is guessing unless you tell it which of the two you are.
Links always cost 23 characters
However long the URL, X counts it as 23. A post that looks 40 characters over can be under once the link is counted properly.
Echoia does the rest: publishing to ten networks, every comment and message in one inbox. Free for three accounts.