Thomas also told the rally that Reform could beat expectations and win a majority in the Senedd, which would require winning at least 49 seats.
Opinion polling for the election have led most to expect that no party will win that many seats the 96-member Welsh Parliament.
Questioned by BBC Wales on whether he has a path to power, Thomas said it would be to “win a majority”.
“The only reason people are saying you can’t win a majority is the opinion polls,” he said.
“I think they’re out. The pollsters are struggling with this system, and with politics in general.”
He added: “I’m not saying we will. I can’t take anything for granted.”
Experts have expected that whoever wins the election will need to find someone else to help get crucial votes through the Senedd, whether in a coalition or a more informal arrangement.
A Plaid Cymru spokesperson said: “If this is all Farage has to say, then it shows just how desperate Reform UK are just two days before a historic Senedd election.
“After all, his party is simply made of parachuted ex-Tories whom bear no reality of the issues facing the communities they seek to represent, nor any credible plans to fix them.”
It added that Plaid was “proud to have candidates rooted in our communities”.