It's not clear which rally you're referring to, so it's not possible to fact check your statement. Trump routinely lies about crowd sizes and his supporters often uncritically repeat those lies, or just assume that Trump draws huge crowds.
There's a lot of things I could say, such as New Hampshire being more about retail poltiics, where candidates routinely do small meetings of a few dozen people. But the big thing is, as Trumpkins should have learned in 2016, that crowds at a rally don't mean anything with regard to vote totals. There's going to be over 130 million votes cast in the 2020 election. That means that 30,000 is 0.02% of the total. It's infinitesimal. All that a rally means is that people are willing to show up for a rally.
What you should be paying attention to are poll numbers. They have Biden straight up trouncing Trump. IIRC the Fox News poll which was just released has Trump losing by twelve points to Biden. Even among voters who dislike both Trump and Biden, Biden beats Trump by double digits.