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Warning levels can be set on most platforms, to log messages when the prefixes received go over a certain threshold. For example, "Shut down the BGP session at 10,000 routes and begin logging warning when the session reaches 80% of that number." (Note that the peering DB currently recommends a 95% warning threshold.)
It is absolutely critical that the session limits be monitored, either through the use of the warning log messages or through other means. The limits are meant to protect from unexpected sudden increases. The slow gradual growth of prefixes received, should they eventually grow enough to reach the limits, can cause an unwatched session to shut down. This sort of "false positive" would be an unfortunate outcome of a technology meant to protect.
You may also want to consider what would happen if your session did shut down. If you primarily rely on one provider, or a default provider, perhaps you would NOT want to shut down the session even if they did flood your BGP with routes. You may also want to consider an auto-restart timer, reenabling the session after 15 minutes or so.
And don't forget your IPv6 sessions!
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