As shown in the graph, Mexico has * over double * number of Spanish speakers of any other countries.
Where is the correct place of definite article (the) in above sentence? I added asterisks as place holders for the definite articles.
In this case you would put "the" before "number."
There are cases where a definite article comes before adverbs and adjectives, such as in the sentence:
Mexico has the most Spanish speakers of any country.
The way you determine where to put the definite article is by determining whether the noun requires a definite article without the modifiers.
Since you wouldn't say
Mexico has number of Spanish speakers
you need a definite article before the noun. If the noun doesn't require a definite article, as in the phrase:
Mexico has Spanish speakers
then you use a definite article to refer to the modifier itself, as in "Mexico has the most Spanish speakers."
So in your case, since "number" requires a definite article, you put it right before the noun.
Mexico has over double the number of Spanish speakers...