Issues with rounding
The maximum number of agents to be used is 5, if you use your agents as soon as they appear then none will be wasted. However the numbers at the top of the screen are truncated to their integer value. The true value can be found by hovering the mouse over the agents to activate a tooltip which shows the current numbers and the amount of agents gained per year. If you have 4.80 Agents and gain more than 0.20 per month then the excess will be wasted and you will not be able to spend it.
Simply keep an eye on the exact numbers, by hovering your mouse on the agents. Don't allow the monthly addition push you over 5 and no agents will be wasted.
Rule of N
Find the number of agents you gain per year and divide by 12 to give the monthly gain. Subtract this number from 5 and round down to the nearest integer. When the agent number is at that level then you should send them. For example gaining 16 merchants per year is 1.25 per month. The next step is 5 minus 1.25 which gives 3.75, rounding down gives 3. Therefore if you gain 16 merchants a year and don't want to waste any you should send them as soon as the value is above 3.
In general the rule of N can be done in portions of 12. If you have less than 12 per year then send at 4, if you have 12-23 then send at 3, if you have 24-35 then send at 2.