If you have Homebrew (which you can get from https://brew.sh), just run:
brew install protobuf
or
PROTOC_VERSION=3.9.1
PROTOC_ZIP=protobuf-all-${PROTOC_VERSION}.zip
curl -OL https://github.com/google/protobuf/releases/download/$PROTOC_VERSION/$PROTOC_ZIP
sudo unzip -o $PROTOC_ZIP -d /usr/local bin/protoc
sudo unzip -o $PROTOC_ZIP -d /usr/local include/*
rm -f $PROTOC_ZIP