There is no explicit limit on skill levels in the rulebook that I can find. (There are for Will, Health, Nature, Circles, and Resources.)
There is an implied limit on p. 33; it labels skills of eves 1-6.
Nature
limited to 7: p31.
Advancement dots on CS r17: 7p 6f = sufficient to get to 8
Will
limited to 6: p31
Advancement dots on CS r17: 6p 5f = sufficient to get to 7
Health
limited to 6: p31
Advancement dots on CS r17: 6p 5f = sufficient to get to 7
Circles
limited to 10: p32
Advancement dots on CS r17: 9p 9f = sufficient to get to 10
Resources
limited to 10: p32
Advancement dots on CS r17: 9p 9f = sufficient to get to 10
Skills
Highest labeled rating: 6 on p 33
Advancement dots on CS r17: 6p and 5f = sufficient to get to 7