There generally aren’t any limits in place for how many entries any individual person can buy for most lotteries.
However, the motto that generally applies to gambling also applies here: “Know your limit and play within it.”
That being said, most of the payment portals that online lottery sites use have a limit on how many can be purchased in one batch, and that is usually in the 20 to 30 ticket range.
Of course, if you need more tickets than that, you can just buy multiple batches, as there are no caps on batches.