Q: If I rip my CDs using iTunes in MP3 format will there be any restrictions on the number of CDs I can burn from those MP3 files?
A: Absolutely none. In fact, regardless of whether your rip your CDs in MP3 format or AAC format, there are no restrictions placed on these. Only files that are purchased from the iTunes Store have any kind of restrictions on them, and in this case even this only applies to normal iTunes Store purchases—“iTunes Plus” tracks are free of any DRM restrictions as well and can therefore be burned to CD as many times as you like and freely converted to any other format.
In fact, even with DRM-protected iTunes Store purchases, you can still burn an individual track to CD an unlimited number of times as long as it is burned from different playlists. The iTunes restriction on burning CDs only applies to complete playlists and limits these to 7 burns. This is presumably intended to prevent somebody from mass-producing copies of a given album or mix CD.
Note that iTunes Plus tracks, even though they do not contain DRM, still contain tags that identify the original purchaser of the track and their iTunes Store account. These tags, however, will not be present if you convert these files to another format (ie, MP3) or burn them to CD.
Also keep in mind that although you can burn an audio CD from any iTunes-supported format, burning an “MP3 CD” from iTunes requires the original files to be in MP3 format—iTunes will not automatically convert other formats to MP3 for you when burning an MP3 CD.