There is no preset limit on the number of fields that you can define in Solr. You will be limited by the resources that you allow Solr to access; RAM, disk etc. There are also cache settings to take into consideration. But when you mention numbers like 100K-1000K, I would suggest that you look at the design of your collections instead.
The limit in size for a particular field varies with the type of field.
Here is a link to the solr wiki describing built-in solr fields.
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