A 518 will not limit you at any school in the context of an otherwise extraordinary application. On here there is bias for stats, just keep in mind however that statistically there cannot be more than 1600 applicants with 520+, only 800 of those are 521+, only around 200 of those are 524+. SDN has sampling bias.
However, even with such small numbers, also keep in mind that the number of T20 seats is limited. There are 21,000 seats total, and the T20 schools make up around 2600 of those seats. This means that you could theoretically fill up every T20 with 520+ applicants and only have around 1000 seats left. Obviously this doesn't happen, but keep that in mind.
To further address your concern, 518 will only 'limit' you at:
Columbia
Harvard
Sinia
Hopkins
Mayo
NYU
Northwestern
Penn
Stanford
UChicago
UVA
Vanderbilt
WashU
Cornell
Yale
Each of these 15 schools have an accepted median at 519 or higher. Keep in mind, however, this list of 15 schools ALL have a 10th% of 518 or lower as well. Also keep in mind, 9 of these schools have matriculated medians lower than the accepted median. This indicates a lot of overlap in the multiple acceptances. So, realistically, your 518 falls below the matriculant median of only 10 schools. So only 10 schools have higher stats than you do. Even then, those 10 schools will still consider you as a strong applicant.
Tl;dr - Don't worry, you good. Apply to 5 T10s, another 10 T20s, and 5 other Einstein/hofstra level schools.