The limitation are those of the NFL. Linebackers are indeed given numbers 50-59 and 90-99 as alternatives. Its Rule 5, Section 3, Article 3c
http://www.jt-sw.com/football/pro/in...ts/0-jers-nums
The only issue I have with this though is that there are players that change positions and keep their numbers. Example: Devin Hester was drafted originally as a CB and thus given number 23, but now he plays WR but he kept his number. There are like 5 other players that have done the same as well.