I LOVE Pronamel toothpaste! I've had teeth softening issues before and started using Pronamel and most of those issues have disappeared. Pronamel is made by Sensodyne so the Sensodyne toothpaste works great too, for sensitivity issues. Good luck!
The best toothpaste here in the UK is usually reckoned to be Oral-B Pro (and it does make your teeth feel lovely and polished). The downside is that it is twice the price of "ordinary" toothpastes.
It is a smidge less than 5.
It is a number. 4 and 875 thousands.