Please answer a question about the following article about Estonian language:

The two different historical Estonian languages (sometimes considered dialects), the North and South Estonian languages, are based on the ancestors of modern Estonians' migration into the territory of Estonia in at least two different waves, both groups speaking considerably different Finnic vernaculars. Modern standard Estonian has evolved on the basis of the dialects of Northern Estonia.

What was the minimum number of waves through which modern Estonians migrated into Estonia?
two