Why not "Je dois pratiquer beaucoup"?

Placement of adverbs follows certain guidelines which often also leave some leeway, depending on what particularly you want to stress in a sentence. Sometimes, there simply is more than one possibility.

In the sentence you quote the verb is actually "dois". This example actually follows nicely the given rule, which specifies that the adverb often follows the (conjugated) verb.

Here is a page which may help to shed some light on this: https://www.dummies.com/languages/french/how-to-position-french-adverbs-in-sentences/