A common issue we have seen people run into is the limit that Shopify imposes on the amount of variants (SKU's) that can be uploaded per day.
A Shopify store can only upload 1000 variants per day. This limit does not apply to Shopify Plus stores.
You can find the information more information from Shopify here.
To reduce the chance of crossing the threshold, if you need to upload a significant amount of products in one day, is to reduce the amount of colors and / or sizes that are offered for a certain product.