The announcement from his lawyer came as the Tory peer reached a £125,000 settlement with ITV and Phillip Schofield.
Andrew Reid declined to name the individuals, but confirmed they could face damages claims reaching thousands of pounds. He also said he had spoken to Scotland Yard about Twitter users who "maliciously" re-tweeted the peer's name.
When asked what action would be taken, he said: "I think most importantly we get an apology and an undertaking not to repeat and, once we've examined the extent of the damage they've done, we'll agree suitable damages."
Asked how many they were seeking damages from, he said: "There are a large amount but at this stage we're looking at 20.
"Twenty high-profile tweeters. They haven't come forward to us. Some of them we're aware of, some of them have come forward and some have been very apologetic.
"All of this will be borne in mind, in the same way that ITV were apologetic, as were the BBC."
Earlier, in a joint statement with This Morning's Schofield, ITV said it had agreed to settle the peer's libel claim.
The broadcaster sparked controversy after Schofield brandished a list of names of alleged abusers he had found on the internet and handed it to Prime Minister David Cameron.
Lord McAlpine reached a £185,000 settlement with the BBC after he was implicated in a botched Newsnight investigation into child abuse.