Where can I get a list of new business registrations?

Your state's Secretary of State publishes them, usually searchable by filing date, and many states offer a bulk download or a paid data service. Start there rather than with a lead tool, because this is one of the few kinds of lead where the government source is better than anything built on top of it.

Can ZipLead tell me which businesses opened this month?

No. It can show you businesses Google has flagged as opening soon, which is the opposite end of the same question and often more useful, and it can say how long a business has been trading when it saw that flag go by. It has no way to date a business that was already open, and it does not guess from review counts.

Is a low review count a good proxy for a new business?

It is a weak one. It catches new businesses and it also catches long-established ones in categories nobody reviews, which for some trades is most of them. If you use it, use it knowing that.

How to find businesses that just opened

How to find newly opened local businesses: the free public records that actually list them, why Google cannot tell you how old a business is, and what a low review count does and does not prove.

THE SHORT ANSWER

The reliable free sources are government filings, not maps: your state's new business entity registrations, your city or county business license list, and county fictitious business name filings, all of which are public and most of which publish weekly. Maps data cannot give you this, because Google only shows an opening date for places that have not opened yet, and a low review count is a weak proxy that also catches ten-year-old businesses nobody reviews.

The sources that actually list new businesses

A newly opened business has to tell the government before it tells Google. That paperwork is public, and it is the only source that is genuinely a list of new businesses rather than a guess at one.

State business entity filings. Every state's Secretary of State publishes new LLC and corporation registrations, most of them searchable by date. This catches the business before it has a sign up, which is early enough to be annoying if you call too soon.
City and county business license lists. Many publish a monthly or weekly list of new licenses issued, sometimes as an open data download. This is closer to the doors opening than a state filing is.
County fictitious business name or DBA filings, which a business has to file before trading under a name that is not its legal one. These are often published in a local newspaper's legal notices, which is why those notices exist.
Alcohol, food service and contractor license boards, if you sell to those trades specifically. A new liquor license or a new food facility permit is a restaurant about to open.

The work in all of these is the same: the filing gives you a name and an address, often a registered agent rather than a real premises, and no phone number and no website. You have to match it to the actual business yourself.

Why maps data cannot answer this

This is worth saying plainly because a lot of tools imply otherwise. Google's Places data carries an opening date only for a business whose status is still future opening, and its own documentation says so. Once the doors are open the field goes away. There is no field anywhere in that data that says how old a trading business is.

Review counts are the usual workaround and they are unreliable in both directions. A brand new restaurant can collect forty reviews in a month. A twenty-year-old machine shop can have three. If a tool sells you "new businesses" and all it is doing is sorting by review count ascending, you should know that is what you are buying.

What ZipLead can honestly do here

Less than you want, and it says so rather than dressing up a proxy. Two real things:

Google flags places that have not opened yet, within about a ninety day window, and ZipLead surfaces that flag with the date. Those are the most useful prospects on this whole page if you sell anything a business buys before it opens: a website, signage, a point of sale system, insurance.
If ZipLead saw a business while it was still flagged as opening soon, it can later tell you how long it has been trading, because the date came from Google at the time. It will not do that for a business that was already open when we first saw it, because there is nothing honest to derive it from.

Everything else on a ZipLead lead is about whether a business is worth calling today rather than how old it is: the score out of 99, the site and certificate checks, whether they are hiring, what software is on their pages, and on licensed trades in California and Texas the date the license was first issued, which is a genuine "in business since" for those two states.

If new businesses are your whole strategy, work the government filings and use ZipLead to enrich and score what you find. It will import a CSV of names and match them to the real businesses, and hand back the rows it cannot place by name rather than quietly mismatching them.

What ZipLead does not do here

Every one of these is a real gap rather than a hedge. If one of them is the thing you actually need, ZipLead is the wrong buy for this job and it is cheaper for both of us if you know that now.

ZipLead has no feed of new business registrations, from any state. It does not read Secretary of State filings, city license lists or DBA notices.
It cannot tell you the age of a business that was already trading the first time it saw it. Nothing in the data supports that, so it reports nothing.
The opening date only exists for places Google has flagged as not yet open, and that flag goes stale. ZipLead ignores one more than a year old rather than announcing it.
License issue dates are California and Texas only, and only for the licensed trades those boards cover.