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This is kind of an open ended question which really the answer is you only need to open the ports you intend to use.
For example you will want port 22 open to communicate with your droplet via SSH. If you change your ssh default port you will need to update UFW to reflect that change.
Second you will need to open ports for services you intend to offer. ie. web and web ssl you will need ports 80 & 443 open respectively.
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