X-Men Legacy published issued #300. Presumably, the prior issue was #299. However that is not the "Marvel Way".
X-Men Legacy (2008) - this is "volume 1" of X-Men Legacy, and it runs from issue #208 to #275, which makes perfect "Marvel Sense". The numbering follows X-Men (2004) and continues in a new X-Men Legacy series in 2013.
X-Men (2004), as you may have guessed by now, ran from #157 to #207, which makes perfect sense - it actually inherited the numbering from New X-Men (2001), which, you may assume, had a #1 issue. But you'd be wrong!
New X-Men (2001) inherited X-Men (1991)'s numbering. This one actually started with #1, which, in the old days, was where sequential counting started. I think this series used to be called "Adjectiveless X-Men" to distinguish from the longer running Uncanny X-Men (1963).
Title | Start | End | Total |
X-Men (1991) | 1 | 113 | 113 |
New X-Men (2001) | 114 | 156 | 156 |
X-Men (2004) | 157 | 207 | 207 |
X-Men Legacy (2008) | 208 | 275 | 275 |
X-Men Legacy (2013) | 1 | 24 | 299 |
X-Men Legacy (2014) | 300+ |