Stolarek et al: Genetic history of East-Central Europe...

[Dewsloth]

"High X-chromosome-based and mtDNA-based contamination was detected in 2 and 15 samples, respectively (Additional file 2: Table S1), which were excluded from further analysis."

Here's a list of contaminated samples:
PCA0050
PCA0117
PCA0122
PCA0143
PCA0150
PCA0191
PCA0324
PCA0332
PCA0333
PCA0390
PCA0403
PCA0406
PCA0496
PCA0497
PCA0506
PCA0554
PCA0572

Source: "Genetic history of East-Central Europe in the first millennium CE"

Do you know why these samples were uploaded to FTDNA if they are contaminated?

oh snap! this deals with four L1029s

incl the sofar only ancient YP444 Lad 191 aswell as YP619 Srodka 554 and also Wielbarks Pruszcz Gdanski 496; so that leaves Balczewo 551 as the only legit ancient YP619 sofar and Kowalewko 38 and Czarnowko 547 as the two legit Wielbarks L1029s; on the otherhand FTDNA does not seem to have incl Lad 213 who is R1a1a1b1a1a1c1 L1029+ in Table S3

how many 'Piast State Slavs' does FTDNA have in its database from the Stolarek paper in total/altogether ?

Well at least the sole Polish DF19 in the study survived that purge, even though he's terrible quality and FTDNA only calls DF19>DF88 (pre-Beaker):

Groszowice 299 was a man who lived between 1000 - 1200 CE during the Late Medieval Age and was found in the region now known as Groszowice, Opole, Poland.

He was associated with the Piast State Slavs cultural group.

His direct maternal line belonged to mtDNA haplogroup K1a.

Reference: PCA0299 from Stolarek et al. 2023

Note:  IIRC, He looks more like a Swede than a Slav autosomally.