I for one is/am

Hello, this is my first post.
Which one is correct to use ‘I for one is’ or ‘I for one am’?
Is that correct to say ‘They for one are’? What i don’t get is the use of ‘is’ after ‘I for one’.

“For one” is an apposition to “I” here: you could even separate it by commas in certain situations: “I, for one, am…”. The pronoun “I” is the subject of the sentence; that’s why it must be “am” here. “For one” is best not used with plural nouns or plural pronouns.