Hi ! You can load all your parquet files this way:
datasets = load_dataset("path/to/the/directory/containing/your/parquet/files")

It does two things:

it converts all the parquet files into Arrow files, and store them in the datasets cache
it opens the Arrow files using memory-mapping - that means that the part of your disk that contains the data is going to be used as virtual memory to load the dataset. Therefore it will load all your data without filling up your RAM.

Then once your dataset is loaded you can tokenize it:
tokenized_datasets = datasets.map(
    tokenize_function, batched=True, num_proc=4, remove_columns=["text"]
)