I don't think that there can be such a concept as an "acceptable" defect arrival rate, in the sense of a target number as you typically want to discover them as fast as possible. So, in english, you want to find as many bugs as you can.
The key way to read that metric is that you want a consistent arrival rate over time, i.e. you want the defect arrival trend to be fairly flat and never zero, say between 15-25 per day as opposed to 2 one day and 50 the next.
Once you have your bugs being discovered consistently, you want to look at the bug fix rate.
The simple answer is to average the number of bugs fixed per day, and divide the total number of bugs by the average. That is approximately how many days until you reach zero bugs. So, if you are fixing 5 bugs per day and you have 200 active bugs, the earliest that you will ship is in 40 working days time. If you want to ship sooner, you will need to stop adding features and focus on fixing more bugs.
The same information can be used in reverse to calculate a maximum allowable bug count. Say you only have 40 days until your desired ship date, and you are fixing 5 bugs per day as in the previous example. If you active bug count is over 200 today, you will probably miss your target. This number continuously decreases so in 2 weeks time, with 30 working days to go, your bug count should be at the 150 mark if you are going to hit your ship date.