HMRC just published their annual report on Creatives Industries, which includes some very interesting data on Theatre Tax Relief.

https://www.gov.uk/government/statistics/creative-industries-statistics-july-2018
Some excerpts from the document:
Since the relief was introduced, £137 million in TTR has been paid out relating to 1,670 claims, which represents 4,680 productions.
Number of productions’ Theatre Tax Relief claims for Touring vs Non-touring productions in 2017-2018
A majority of Theatre Tax Relief claims are for non-touring productions.
Source: HMRC Management Information Systems (MIS)
In 2017-18, £77 million was paid out relating to 910 claims. Since each claim may relate to more than one production, this represents a total of 2,980 productions, of which 1,140 were touring and 1,840 were non-touring.
Number of Theatre Tax Relief Claims by size in 2017-2018
Source: HMRC Management Information Systems (MIS)
A Theatre Tax Relief Claim is annual (through annual corporation tax return) and one single claim can aggregate several productions. E.g. a claim fo £150k could be for 3 productions for an average of £50k per production.
The majority of claims are for smaller amounts, with 36% of all claims being for £10,000 or less in 2017-18. These claims however only represent 2% of the total amount paid out.
In 2017-18, claims over £250,000 represented just 8% of the number of claims made but 64% of the total amount paid out.
Claimed vs Paid success rate for Theatre Tax Relief Claims in 2016-2017
84% of the productions’ claims were successful in 2016-2017
Source: HMRC Management Information Systems (MIS)