増える and 増やす: a higher count and a plan to add more
Distinguish a rising count from an action to increase it, and track initial and final quantities, including doubling.
More people or items
週末になると、この店は客が増えます。 says that this shop gets more customers on weekends. 増える, read ふえる, describes a quantity becoming larger.
登録者が百人から百二十人に増えました。 means that the number of registered people rose from one hundred to one hundred twenty. The increase is twenty; the final count is one hundred twenty.
Decide to increase supply
店は週末のスタッフを増やしました。 says that the shop increased its weekend staffing. 増やす, read ふやす, marks the increased item with を.
練習の回数を週一回から二回に増やします。 means that practice will increase from once to twice a week. Distinguish the one added session from the final schedule of two per week.
Multiples and unrealized plans
席を二倍に増やしました。 means that the number of seats was doubled. Ten original seats would become twenty, not thirty.
増やす予定ですが、まだ増えていません。 says there is a plan to increase the amount, but it has not increased yet. Separate the plan from the present result; the te-form of 増やす is 増やして.
If ten seats are increased as 二倍に増やしました, how many seats are there afterward?
Show the answer and why
Twenty. 二倍に makes the final count twice the original; it does not mean adding twice the original on top of it.