There are a lot of modules which aim to limit the number of nodes that a user/role can create. Here's a brief roundup (please add more types or qualifications via comments).
Node limitnumber
- 5.x, 6.x only
- 6.x-2.x relies on rules
- Limits can be created using rules.
- Time-intervals are now implemented
- Under active development
create quota
- Relies on rules
- 6.x only
- Claims to be extensible
- Seems to require the implementer to write some code
- No longer maintained
user_quota
- Limits per user, per content type.
- Users who are not limited on a content type can create as many of that type as they want.
- Admin screen shows a history of each alteration of a user's quota and allows admins with "administer site configuration" to add (or subtract) from the quota.
nodefamily
- 4.7, 5.x only
- Creates relationships between nodes
- Limits the number of nodes of a type that a user can create
node_limit
- 6.x only
- Allows for per-user, per-role, per-og, per-timeframe, per-timeinterval, per-taxonomyterm, etc limits (or any combination thereof)
I have actually built yet another one of these which has limits per user per content type. If there is no limit for a uid/type then it allows the content to be created. If folks agree this is a not a duplicate of the existing nodes I'll contribute it and document it's difference from the existing solutions ;)