C’est un nombre qui symbolise la similarité des objets i et j.

The translation Natashacycle-r proposes is correct and is a word-for-word transposition of your original sentence.

C’est un nombre qui symbolise la similarité des objets i et j.

I am however of the mind that word-for-word translation are often poor or heavy. Heavy in this case. Just a hint to start making things easier on the eyes:

Ce nombre symbolise la similarité des objets i et j.

Now, being short of context, it is hard to find the most relevant translation. I am going to make some assumptions here.

In French, “symbolize” somehow implies that the value is a symbol, an approximation. I might be wrong (though the first definition for “symbolize” on Wiktionary seems to support my point) but I think “symbolize” can be used in English even for precise measures. If it is the case here, I would suggest the following:

Ce nombre mesure la similarité des objets i et j.

To accurately translate, the context is often important. What is the general area (math, physics, …)? What number are you defining? What type of objets are i and j? Do not hesitate to give additional information rather than a raw sentence when you need help translating, as it will almost always help giving you better answers.

Have you used “similitude” already in your text? although it is the same structure therefore… similar..

c’est un nombre qui represente la similitude entre i & j

might be better:

c’est un nombre qui [expose || montre || démontre] l’affinité entre i & j.