Two is the most frequent (spelled frequent ) sounds fine to me.
Alternatively, you could write something like
The number two is the most frequent value in the given array.
I think the "one word" you are looking for is mode .
It is mathematical terminology.
In the given array, two is the mode .
mode
The mode is the value that appears most often in a set of data.
(Wikipedia)
ell.stackexchange.com/users/36187/Em.
" Most frequent " seems reasonable to me.
If you really need a single word for some reason, you could choose commonest : commonest superlative form of common :
most common.
common Occurring or happening regularly or frequently; usual. For a mathematically-rigorous word
, you should use mode : mode ( statistics )
The most frequently occurring value in a distribution.
ell.stackexchange.com/users/44500/Toby Speight
Some more suitable words: Predominant, Preponderant, Principal.
All can be used to mean "most frequent", and are better known than "mode" (
which is rarely if ever used by anyone who isn't a mathematician) while also sounding more professional than something like "commonest".
E.g. "The preponderant number is 2".
ell.stackexchange.com/users/18154/Benubird
You could say " prevalent " or " abound "
For arrays the fixed phrase
"most frequent value" is widely used.
In mathematics the term "mode" is common.
Here's a good explanation of what a mode is - Most frequent values in array .
ell.stackexchange.com/users/44134/SovereignSun
As the question mentions an array of numbers, seems to me mode is absolutely the correct answer (and had I got here quicker
I think that would have been my one-word answer).
This used to be taught at primary level in the UK (until 2015), so not obscure terminology :)
ell.stackexchange.com/users/55730/Will Crawford
Let's say I have the next array of 9 numbers: 1, 2, 2, 4, 2, 2, 3, 2, 1. I want to say that 2 has the highest frequency in the array. But I thought I could write this in one word. I also thought of "2 is the most frequent". Which is the best way to say that a number is the most frequent in an array?
How to say "this number is the most frequent" in one word?
ell.stackexchange.com
<word-request>
ell.stackexchange.com/questions/130131
ell.stackexchange.com/users/55076/Timʘtei
Predominant, preponderant, principal, prevalent are all suggested. However, in mathematics the term  "mode"  is common. Some more suitable words: predominant, preponderant, principal. All can be used to mean "most frequent"
Some more suitable words: Predominant, Preponderant, Principal. All can be used to mean "most frequent", and are better known than " Mode" is rarely if ever used by anyone who isn't a mathematician.