The Last Jedi Visual Dictionary says this about the First Order Fleet (relevant parts are bold):
The discovery of the Resistance base's location leads to the dramatic
arrival of the First Order fleet at D'Qar. Spearheading the assault is
the Finalizer, the Resurgent-class Star Destroyer under General Hux's
direct command. Even this overwhelming force is merely the first
wave of a larger offensive. The true scale of the First Order military
is beyond even General Organa's worst-case projections. In the
shadowy corners of uncharted space - and within the hidden ledgers of
scheming weapons manufacturers - the First Order has been secretly
building for war. With the New Republic fleet vaporized by the
Starkiller, it now stands unopposed.
So, there's no army big enough to challenge the First Order now that the New Republic's fleet is gone. It's also significant that this attack is only the first wave. So even the overwhelming force that arrives at D'Qar is just a piece of the entire First Order's army.
For some extra perspective, The Last Jedi: Incredible Cross-Sections says that the Dreadnought has a crew of 215,000 (53,000 officers, 140,000 enlisted, and 22,000 stormtroopers). Poe says "We have a chance to destroy a Dreadnought", implying that there are others - so there's at least 400k+ people fighting with the First Order, compared to the 400 people that make up the Resistance (according to a line from Vice-Admiral Holdo).
This is related and interesting, but not actually part of the answer to this question:
The Force Awakens opening crawl says:
Luke Skywalker has vanished. In his absence, the sinister FIRST ORDER
has risen from the ashes of the Empire and will not rest until
Skywalker, the last Jedi, has been destroyed.
This tells that The First Order did not arise immediately after the Empire fell. Rather, it came into existence after Luke went into hiding. We know he didn't leave until after his conflict with Ben Solo, so it was at least 16-ish years later (the age of Ben Solo when he turned on Luke).