Can you please tell me how you have 5 toolbelts. 4 is the best that I've been able to obtain.
Regards,
~B

There's several mods that add toolbelts. In the particular save that this screenshot came from, it used:

• Base game research (adds 1 toolbar)
• My own mod QoL research (adds 2 toolsbars)
• Bobs Logistics (I believe also adds 2 toolbars)

I chose this 5 toolbars, to show that it works with odd numbers of toolbars too, cheers!

Through LUA, I found that a total of 8 can be added:

/c game.player.quickbar_count_bonus=3
/c game.player.force.quickbar_count=5

Both gives 8 toolbelts

That's not entirely correct. To clarify:

/c game.player.force.quickbar_count = X
Would give each player on your force, X quickbars. So if you just wanted all players in your game to have 8 quickbars, you could just do:
/c game.player.force.quickbar_count = 8

/c game.player.quickbar_count_bonus = X
Will give an individual player, X bonus quickbars.

So the total number of quickbars for any player will be:
player.force.quickbar_count + player.quickbar_count_bonus

Note that setting either of these may conflict with a mod's behavior. This mod only uses the quickbar_count on force. You could create a crash using this method:

1. Research both toolbelt upgrades from this mod.
2. Run /c game.player.force.quickbar_count = 1
3. Set the modifier setting for quickbars to 0.5.

The mod then figures out that it has to go from 2 quickbars to 1 (0.5 multiplier), it will then run force.quickbar_count = force.quickbar_count - 1, setting it from 1 to 0, which crashes the game. I could prevent it from crashing, but realistically seen, nobody will run into this scenario, and if you do, it's kind of your own fault.

My point is, be a bit careful when messing with these properties in conjunction with mods that modify these values.