At least three in my experience... and I'm not even sure that they are accurate.
I bought the first thermometer at Walgreens -- it was their store brand and the only model they seem to carry. It was around $10.00. I used this one for about 2 days, when it started to act like it needed a new battery.
So, instead, I ordered this one : http://www.early-pregnancy-tests.com/bbt-thermometers.html
It also worked for only a few days. Yesterday morning, it seemed like it was taking forever for the thermometer to beep. So, I look and the damn thing shut off. I turn it back on stick it in my mouth, and cross my eyes to see the display. I watch it count up, pause, and turn off. WTF???
I'm one of those people who can never have a cell phone, cordless phone, or watch for any length of time without killing the batteries. Seriously though, I haven't had these things for more than 2 weeks... surely that is not the cause.
Does anyone know a good brand of basal thermometer? Basal thermometers read like this 98.60 instead of 98.6. Apparently the extra digit makes all the difference.
The thing I'm most peeved about is the fact that now I'm not sure if the huge drop in temperature yesterday was due to the use of the walgreens thermometer instead of the one I bought online. Who knows. Oh, and I replaced the battery in the Walgreens one, but they are made so cheaply that the battery doesn't stay seated and that is what was causing my low battery signal. It wasn't making a full connection. Good.
I'm now down $23.00 and still I don't have a consistently working thermometer.