Build-Bot queue individual fluctuate rapidly but overall-sum locks at 601

Solution 1:

There is no “queue”. What you are seeing is a list of alerts only.

In order to ensure consistent performance, Factorio does not reevaluate the entire world's possible construction requests all at once. Instead, it checks one build ghost at a time, and checks whether an item and bot are available. If they are, the task is assigned to the bot; if not, an alert is generated. There is a limit of at most one alert generation per tick, and they last for 10 seconds = 600 ticks, so can only ever be about 600 construction alerts visible at once.

The lifetime of alerts has nothing to do with how bots are dispatched. There is no harm to your bots' performance from this display fluctuation.

If you want the alerts to be processed faster than the default 1 per tick, thus allowing more alerts to be displayed, you could change the internal setting like so:

/c game.player.force.max_failed_attempts_per_tick_per_construction_queue = 2

to get 1200 alerts, or a higher number for more.

Note that this will count as a "cheat" and disable achievements. I found it in this Factorio forum thread.