The translation @cl-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.