The number 0 is displayed rei but is pronouced zero, is it normal ?
Many kanji can be pronounced in different ways in different situations. Those pronouncations are called “readings”. There are two major groups of readings called “on’yomi” and “kun’yomi”.
The background is that for a long time, Japanese didn’t have a writing system. Japanese could be spoken, but not written down. Chinese, on the other hand, already had a writing system. At some point in time, Chinese scholars traveled to Japan and brought their writing system with them.
Now, the Japanese scholars were confronted with a problem. The Japanese and the Chinese language were quite different. Should they read the new, Chinese characters as they were pronounced in Chinese, or should they read the characters like the meaning in Japanese was pronounced?
To illustrate the dilemma, let’s imagine French had no writing system. Then, somebody from England came over the channel and introduced the French to that newfangled thing called “writing”. Let’s say, in English, the symbol 木 stood for “tree”. So, how should the French pronounce 木? Should they say “tree”, because that’s how the symbol is pronounced, regardless of the meaning? Or should they say “arbre”, because that’s what they are calling that wooden thing with leafs in their language?
The ever polite Japanese basically did both 
So, many of the Japanese characters adapted from Chinese characters (that is, the kanji) have multiple readings. The kun’yomi are readings that descend from the Japanese pronounciations, the on’yomi are readings that descend from the Chinese pronounciations. And because things aren’t complex enough yet, many kanji have multiple kun’yomi and/or multiple on’yomi 
As a rule of thumb, when the whole word is made up from one kanji (and no kana), it mostly uses a kun’yomi. For example, if you use the kanji representing the concept of “tree”, 木, to write the word “tree”, you read 木 as き, “ki”. If you use the kanji 木 in a word made up from multiple kanji like 木曜日 (Thursday), you read 木 as もく, “moku”.
To get back to your question, the concept of zero is represented by the kanji 零. One of the kun’yomi of this kanji is ぜろ, “zero”. One of the on’yomi of the kanji is れい, “rei”.
As a side note, it’s common to write kun’yomi in hiragana and on’yomi in kataka. In this post, I only used hiragana so things wouldn’t get too confusing 