How to prepend a string to only the lines of text which are numbers

sed can do that:

$ sed 's/^[[:digit:]]*$/number-&/' input.txt
Series
Of
Word
number-73914
Again
Word

In case we want to account for empty lines, we'd use + and -r option:

$ sed -r 's/^[[:digit:]]+$/number-&/' input.txt
Series
Of
Word
number-73914
Again
Word
line below is empty

line above is empty

Once you verify everything is proper, you can use -i option to edit the file itself, i.e. sed -i .... Otherwise, you can always make a copy of the file with sed 's/^[[:digit:]]*$/number-&/' input.txt > output.txt

Note that this assumes consistent file format, with no leading whitespaces or trailing whitespaces on each line.

And here's Python as extra:

$ python3 -c 'import sys; print("\n".join([ "number-" + i.strip() if i.strip().isnumeric() else i.strip() for i in sys.stdin]))' < input.txt
Series
Of
Word
number-73914
Again
Word
line below is empty

line above is empty

One way using awk:

awk '/^[0-9]+$/{$0="number-"$0;}1'  file

lines that contain ONLY numbers

It's unclear whether you mean numbers or just 0-9. Here's a Perl one-liner that picks out the likes of 123, 3.14 and 1e-12 while ignoring various representations of infinity and not-a-number:

$ perl -MScalar::Util -ne 'chomp; if (!(m/^\s/ || m/^[\+-]?inf(?:inity)?$/i || m/^nan$/i) && Scalar::Util::looks_like_number($_)) { print("N:"); } print("$_\n");' <x
a
N:123
N:+1
N:-1
 1
b

1a
N:3.14
c
3.1415926 is an approximation of pi
N:1e-12
inf
Inf
Infinity
Infinity +1 sword
+Infinity
-infinity
NaN
1/2

I changed the prefix to "N:" simply because "number--1" looks a bit rubbish. Note that this treats " 1", for example, as not numeric. If that is undesirable behaviour, do not include the "m/^\s/" test for leading whitespace.

If you mean "0-9", Sergiy's sed solution above is fine.

You can try this

$cat input.txt
Series
Of
Word
73914
Again
Word

$awk '{ if($1 ~/[0-9]/) printf "number - %s\n",$1; else print $1 }' input.txt
Series
Of
Word
number - 73914
Again
Word