7 MPs would be quite impressive for party that's only existed for 5 years or so. The Green party have been around for decades without getting that number.
And even Farage hasn't claimed he'll have the second largest number of MPs. Reform have been quite clear they have a two election strategy. They want a handful of seats now, which guarantees them a higher profile and a certain level of media coverage, and a larger number of second place finishes. Then as the Labour government inevitably becomes less popular they'll make the "See, we told you. Labour and the Tories are essentially the same. We're the real alternative." argument while also flipping the current Sunak message by telling people not to waste their vote on the Tories. They may also benefit from the voting age dropping to 16. Reform are surprisingly popular among teenagers.